The `INSERTKEY_XYZ_TO_RGB` flag on `datablock.keyframe_insert()` in Blender's Python API was changed to do nothing in #115297 and #119625, in favor of determining fcurve coloring exclusively from user preferences. This PR removes that now-useless `INSERTKEY_XYZ_TO_RGB` flag entirely. Pull Request: https://projects.blender.org/blender/blender/pulls/147262