The generic Action assignment function tries to find a slot to
auto-assign. This would always look at the last-used slot identifier on
the ID's `AnimData` struct, even when assigning to an NLA strip or
Action constraint.
This commit removes the `Action::find_suitable_slot(ID)` method, and
replaces it with a `generic_slot_for_autoassign(ID, action,
last_slot_identifier)` function. That function basically copies the
behaviour of `find_suitable_slot()`, except that it gets the
`last_slot_identifier` from the caller.
Another difference is that it no longer checks whether the Action is
already assigned, and so also never uses the currently-assigned slot
handle. In the only code flow that calls `generic_slot_for_autoassign()`
this situation would never occur, and thus it's better to delete this
dead code.
Pull Request: https://projects.blender.org/blender/blender/pulls/131491
bf9eb57554