To do so, matching BKE 'API' was also refactored a bit:
* Get Pose data instead of Object, as parameter;
* Removed some sanity checks not needed at such a low level (callers are supposed to do that);
* You can now remove an arbitrary bone group, not only the active one.
Based on patch by pkrime (Paolo Acampora), with own edits.
Reviewers: #python, pkrime, aligorith
Reviewed By: aligorith
Differential Revision: https://developer.blender.org/D522