Delete `courier:batchSearches` from settings during 8.0 migration

[Dosant]

Kibana version: 8.0-rc1

Description of the problem including expected versus actual behavior:

We removed courier:batchSearches that was deprecated for a while #109350

When upgrading to 8.0 and when you had this setting ON you'd still have it in the advanced settings screen as a custom setting:

We should remove the setting during 8.0 SO migration:

kibana/src/core/server/ui_settings/saved_objects/migrations.ts

Lines 78 to 100 in c972435

	'8.0.0': (doc: SavedObjectUnsanitizedDoc<any>): SavedObjectSanitizedDoc<any> => ({
	...doc,
	...(doc.attributes && {
	attributes: Object.keys(doc.attributes).reduce(
	(acc, key) =>
	[
	// owner: Team:Geo
	'visualization:regionmap:showWarnings',
	'visualization:tileMap:WMSdefaults',
	'visualization:tileMap:maxPrecision',
	// owner: Team:Core
	'telemetry:optIn',
	'xPackMonitoring:allowReport',
	'theme:version',
	].includes(key)
	? {
	...acc,
	}
	: {
	...acc,
	[key]: doc.attributes[key],
	},
	{}

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServicesSv)