This exposes the fcurve cleanup from transform system to other callers
in anticipation to use it in the snapping operators.
It has been renamed from `posttrans_fcurve_clean` to
`BKE_fcurve_merge_duplicate_keys` to better describe what it does.
No functional change expected.
Ref. T101996
NOTE: same probably has to be done for greasepencil, but that is for
another commit.
Maniphest Tasks: T101996
Differential Revision: https://developer.blender.org/D16663
a179246e1f