Blender Git Loki

Kernel: allow unregistering BKE callback functions

Introduce `BKE_callback_remove()`, which undoes the effect of
`BKE_callback_add()`. It also respects `funcstore->alloc` by freeing the
removed `funcstore` when needed.

This allows for shorter-lived objects in memory to unregister their
callbacks at the end of their lifespan.

`BKE_callback_global_finalize()` has been adjusted so that the
responsibility "remove a callback" is given to one function only.

Reviewed by: campbellbarton

Differential Revision: https://developer.blender.org/D12625