MDL-50491 filters: Some final touches.

MDL-50491 filters: Some final touches.

- Complete the deprecation information to follow the rules.
- Minor tweak to PLAIN tests to have clearer expectations.
- Completely delete get_global_config() from filters, there
  was not much utility keeping it there. Note we have opted
  for direct deletion because it's private API, not public one
  (and searching github/google/docs did not reveal any use
  of those functions either, just in case).