[Monitoring] Remove code related to Beats Central Management

[jen-huang]

BCM was removed in 7.14 (elastic/beats#25696, #99789). There are still leftover references in monitoring code related to .management-beats index that should be cleaned up:

kibana/x-pack/plugins/monitoring/server/lib/setup/collection/get_collection_status.js

Line 239 in 0ae2134

indices: ['*beat-*', '.management-beats*'],

There are other Beats-related code in this file but I am not exactly sure if those should be removed too.

Per @chrisronline:

The only other thing I’d say is this code might be checking the monitoring cluster instead of the production cluster which might be running a different version of the stack. The monitoring cluster should always be >= so it should be okay but just something to consider

[elasticmachine]

Pinging @elastic/stack-monitoring (Team:Monitoring)

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)