Refactor `Action::slot_remove`: implemented iteration over
`strip_keyframe_data_array` and direct calling
`StripKeyFrameData::slot_data_remove()`. Also removed
`Strip::slot_data_remove()` as well as `Layer::slot_data_remove` as it
had no usages after refactoring.
Fixes: #137095
Pull Request: https://projects.blender.org/blender/blender/pulls/138343