`BKE_animdata_fix_paths_remove()` now calls into the new function
`animrig::legacy::action_fcurves_remove()`, which works for both
legacy and slotted Actions.
Also I updated the documentation of the BKE function so that it's clear
what it's actually doing.
No functional changes for legacy Actions.
Ref: #123424
Pull Request: https://projects.blender.org/blender/blender/pulls/128252