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