Remove AsyncDNSResolver

[kpschoedel]

Problem

AsyncDNSResolver is unused, and also uses threading internally,
which interferes with threading cleanup (issue #7725).

Change overview

Remove it.

Testing

CI. (This code had no unit tests.)

[github-actions]

PR #10660: Size comparison from 87c3a1a to d1dad71

10 builds (for k32w, mbed, p6, qpg, telink)

platform	target	config	section	87c3a1a	d1dad71	change	% change
k32w	lock-app	k32w061+debug	.bss	69052	69052	0	0.0
			.data	1864	1864	0	0.0
			.text	515520	515520	0	0.0
	shell	k32w061+debug	.bss	55080	55080	0	0.0
			.data	672	672	0	0.0
			.text	357332	357332	0	0.0
	lighting-app	k32w061+se05x+release	.bss	78576	78576	0	0.0
			.data	1900	1900	0	0.0
			.text	614304	614304	0	0.0
mbed	lighting-app	CY8CPROTO_062_4343W+release	.bss	172164	172164	0	0.0
			.data	5464	5464	0	0.0
			.heap	858816	858816	0	0.0
			.text	1219944	1219944	0	0.0
	lock-app	CY8CPROTO_062_4343W+release	.bss	171084	171084	0	0.0
			.data	5432	5432	0	0.0
			.heap	859928	859928	0	0.0
			.text	1197840	1197840	0	0.0
p6	lock-app	default	.bss	68216	68216	0	0.0
			.data	2416	2416	0	0.0
			.heap	962712	962712	0	0.0
			.text	1126624	1126624	0	0.0
qpg	lighting-app	qpg6100+debug	.bss	53552	53552	0	0.0
			.data	996	996	0	0.0
			.text	486616	486616	0	0.0
	lock-app	qpg6100+debug	.bss	52488	52488	0	0.0
			.data	952	952	0	0.0
			.text	462684	462684	0	0.0
	persistent-storage-app	qpg6100+debug	.bss	17778	17778	0	0.0
			.data	280	280	0	0.0
			.text	102704	102704	0	0.0
telink	lighting-app	tlsr9518adk80d	bss	71000	71000	0	0.0
			noinit	33216	33216	0	0.0
			text	458304	458304	0	0.0

12 builds (for efr32, linux)

platform	target	config	section	87c3a1a	d1dad71	change	% change
efr32	lighting-app	BRD4161A	.bss	118036	118036	0	0.0
			.data	1800	1800	0	0.0
			.text	782944	782944	0	0.0
	lock-app	BRD4161A	.bss	115892	115892	0	0.0
			.data	1760	1760	0	0.0
			.text	762088	762088	0	0.0
	window-app	BRD4161A	.bss	116212	116212	0	0.0
			.data	1764	1764	0	0.0
			.text	763012	763012	0	0.0
	lighting-app	BRD4161A+rpc	.bss	131364	131364	0	0.0
			.data	1852	1852	0	0.0
			.text	762700	762700	0	0.0
linux	all-clusters-app	debug	.bss	52176	52176	0	0.0
			.data	978	978	0	0.0
			.data.rel.ro	58688	58688	0	0.0
			.dynamic	592	592	0	0.0
			.got	4072	4072	0	0.0
			.init	27	27	0	0.0
			.init_array	512	512	0	0.0
			.rodata	136373	136373	0	0.0
			.text	1345186	1345186	0	0.0
	chip-tool	debug	.bss	17552	17552	0	0.0
			.data	1584	1584	0	0.0
			.data.rel.ro	86192	86192	0	0.0
			.dynamic	592	592	0	0.0
			.got	4344	4344	0	0.0
			.init	27	27	0	0.0
			.init_array	416	416	0	0.0
			.rodata	178892	178892	0	0.0
			.text	2859941	2859941	0	0.0
	ota-provider-app	debug	.bss	37472	37472	0	0.0
			.data	752	752	0	0.0
			.data.rel.ro	23176	23176	0	0.0
			.dynamic	592	592	0	0.0
			.got	4008	4008	0	0.0
			.init	27	27	0	0.0
			.init_array	440	440	0	0.0
			.rodata	110056	110056	0	0.0
			.text	1010594	1010594	0	0.0
	ota-requestor-app	debug	.bss	205728	205728	0	0.0
			.data	752	752	0	0.0
			.data.rel.ro	24488	24488	0	0.0
			.dynamic	592	592	0	0.0
			.got	4136	4136	0	0.0
			.init	27	27	0	0.0
			.init_array	512	512	0	0.0
			.rodata	128744	128744	0	0.0
			.text	1130130	1130130	0	0.0
	shell	debug	.bss	16072	16072	0	0.0
			.data	242	242	0	0.0
			.data.rel.ro	35120	35120	0	0.0
			.dynamic	592	592	0	0.0
			.got	3496	3496	0	0.0
			.init	27	27	0	0.0
			.init_array	336	336	0	0.0
			.rodata	71855	71855	0	0.0
			.text	569858	569858	0	0.0
	tv-app	debug	.bss	216400	216400	0	0.0
			.data	2032	2032	0	0.0
			.data.rel.ro	55872	55872	0	0.0
			.dynamic	592	592	0	0.0
			.got	4400	4400	0	0.0
			.init	27	27	0	0.0
			.init_array	608	608	0	0.0
			.rodata	152296	152296	0	0.0
			.text	1464706	1464706	0	0.0
	bridge-app	debug+rpc	.bss	52912	52912	0	0.0
			.data	976	976	0	0.0
			.data.rel.ro	25784	25784	0	0.0
			.dynamic	592	592	0	0.0
			.got	3944	3944	0	0.0
			.init	27	27	0	0.0
			.init_array	400	400	0	0.0
			.rodata	110644	110644	0	0.0
			.text	1052629	1052629	0	0.0
	lighting-app	debug+rpc	.bss	42232	42232	0	0.0
			.data	1106	1106	0	0.0
			.data.rel.ro	52496	52496	0	0.0
			.dynamic	608	608	0	0.0
			.got	4104	4104	0	0.0
			.init	27	27	0	0.0
			.init_array	528	528	0	0.0
			.rodata	127857	127857	0	0.0
			.text	1253906	1253906	0	0.0

12 builds (for esp32, nrfconnect)

platform	target	config	section	87c3a1a	d1dad71	change	% change
esp32	all-clusters-app	c3devkit	.dram0.bss	60296	60296	0	0.0
			.dram0.data	16192	16192	0	0.0
			.flash.rodata	199008	199008	0	0.0
			.flash.text	872638	872638	0	0.0
			.iram0.text	57330	57330	0	0.0
		m5stack	.dram0.bss	62792	62792	0	0.0
			.dram0.data	32084	32084	0	0.0
			.flash.rodata	207432	207432	0	0.0
			.flash.text	902739	902831	92	0.0
			.iram0.text	125115	125115	0	0.0
nrfconnect	lighting-app	nrf52840dk_nrf52840	bss	112332	112332	0	0.0
			rodata	97776	97776	0	0.0
			text	577652	577652	0	0.0
	lock-app	nrf52840dk_nrf52840	bss	111348	111348	0	0.0
			rodata	94112	94112	0	0.0
			text	559152	559152	0	0.0
	pigweed-app	nrf52840dk_nrf52840	bss	51772	51772	0	0.0
			rodata	45772	45772	0	0.0
			text	339392	339392	0	0.0
	pump-app	nrf52840dk_nrf52840	bss	111416	111416	0	0.0
			rodata	95092	95092	0	0.0
			text	562308	562308	0	0.0
	pump-controller-app	nrf52840dk_nrf52840	bss	111356	111356	0	0.0
			rodata	94172	94172	0	0.0
			text	558944	558944	0	0.0
	shell	nrf52840dk_nrf52840	bss	107316	107316	0	0.0
			rodata	71640	71640	0	0.0
			text	519032	519032	0	0.0
	lighting-app	nrf52840dk_nrf52840+rpc	bss	108572	108572	0	0.0
			rodata	88560	88560	0	0.0
			text	550856	550856	0	0.0
		nrf5340dk_nrf5340_cpuapp	bss	113704	113704	0	0.0
			rodata	93016	93016	0	0.0
			text	507116	507116	0	0.0
	lock-app	nrf5340dk_nrf5340_cpuapp	bss	112720	112720	0	0.0
			rodata	89372	89372	0	0.0
			text	488612	488612	0	0.0
	shell	nrf5340dk_nrf5340_cpuapp	bss	108300	108300	0	0.0
			rodata	66284	66284	0	0.0
			text	439632	439632	0	0.0