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.