`BKE_fcurves_filter()` was only used by the Symmetrize Armature
operator, when dealing with Action Constraints on the bones.
`BKE_fcurves_filter()` is now removed, and has been replaced by the new
`animrig::fcurve_find_in_action_slot_filtered()`. It's functionally
pretty much the same, except that it returns a `Vector<FCurve *>`
instead of manipulating a `ListBase` parameter.
Because it's now implemented using an iterator in
`ANIM_action_iterators.hh` it didn't feel right to keep the function in
BKE.
No functional changes.
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