[snmp] removing unneeded go-variant

[truthbk]

What does this PR do?

Removes the SNMP go check and all it's moving parts (ie. net-snmp).

Motivation

Python snmp check now behaves well enough in an A6 multi-runner universe, and is way more maintainable.

Additional Notes

Let's check all is well in the resulting omnibus build, including some python SNMP testing.

[codecov]

Codecov Report

Merging #3164 into master will increase coverage by 0.21%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #3164      +/-   ##
=========================================
+ Coverage   54.38%   54.6%   +0.21%     
=========================================
  Files         541     542       +1     
  Lines       38762   38627     -135     
=========================================
+ Hits        21081   21091      +10     
+ Misses      16425   16289     -136     
+ Partials     1256    1247       -9

Flag	Coverage Δ
#linux	57.66% <ø> (+0.3%)	⬆️
#windows	56.21% <ø> (+0.09%)	⬆️

Impacted Files	Coverage Δ
cmd/agent/app/jmx.go	16.12% <0%> (-0.54%)	⬇️
cmd/agent/app/control_service_windows.go	2.58% <0%> (-0.5%)	⬇️
cmd/agent/app/integrations.go	21.33% <0%> (-0.26%)	⬇️
cmd/agent/app/check.go	5.12% <0%> (-0.22%)	⬇️
cmd/agent/gui/agent.go	0% <0%> (ø)	⬆️
pkg/secrets/info.go	0% <0%> (ø)	⬆️
cmd/agent/app/integrations_windows.go	0% <0%> (ø)	⬆️
pkg/forwarder/transaction.go	82.9% <0%> (ø)	⬆️
cmd/agent/app/dependent_services_windows.go	0% <0%> (ø)	⬆️
pkg/secrets/info_windows.go	65.51% <0%> (ø)
... and 4 more

[arbll]

Apart from a few relevant references in two docs, I think you 🔥 it all.

https://github.com/DataDog/datadog-agent/blob/master/docs/dev/agent_build.md
https://github.com/DataDog/datadog-agent/blob/master/docs/dev/agent_dev_env.md

Feel free to merge once addressed.