No functional changes
Make the code more readable by doing the following
* rename `BKE_keyingset_free` to `BKE_keyingset_free_paths` since that is what it does
* invert `if` and return early to reduce indentation
* add enum value `MODIFYKEY_SUCCESS` to explicitly state the return value, instead of eModifyKey_Returns(0)
* return `INSERTKEY_NOFLAGS` instead of eInsertKeyFlags(0)
* move variables closer to their usage
Pull Request: https://projects.blender.org/blender/blender/pulls/113666
6922cb4683