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).

lib/filterlib.php

@@ -233,7 +233,8 @@ class filter_manager {
     }
 
     /**
-     * @deprecated Since Moodle 3.0. This was used by the old text filtering system, but no more.
+     * @deprecated Since Moodle 3.0 MDL-50491. This was used by the old text filtering system, but no more.
+     * @todo MDL-50632 This will be deleted in Moodle 3.2.
      * @param context $context the context.
      * @return string the hash.
      */
@@ -385,7 +386,8 @@ abstract class moodle_text_filter {
     }
 
     /**
-     * @deprecated Since Moodle 3.0. This was used by the old text filtering system, but no more.
+     * @deprecated Since Moodle 3.0 MDL-50491. This was used by the old text filtering system, but no more.
+     * @todo MDL-50632 This will be deleted in Moodle 3.2.
      * @return string The class name of the current class
      */
     public function hash() {