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