Delete operator was shared for strips and retiming keys, which was not
ideal. Operator logic relied on invoke function setting proper context
which was also hidden from users.
Split these operators and make dedicated operators for removing retiming
keys - `SEQUENCER_OT_retiming_key_delete`.
This operator returns `OPERATOR_CANCELLED | OPERATOR_PASS_THROUGH` if
executed in non-retiming context, or no key is selected.
Pull Request: https://projects.blender.org/blender/blender/pulls/123767
1a887c06d9