This commit essentially:
* Remove the 'delete single ID' code path.
The multi-deletion code has now be around for a long while, so we
should be able to assume it is mature enough. It is also several times
faster when deleting a lot of IDs at once.
* Move away from using `LIB_TAG_DOIT` ID tag in internal code, using a
Set instead to store the IDs to be deleted.
Potential side-effects:
* The 'delete single ID' codepath (removed by this commit) was making
some dangerous assumptions regarding order of IDs in Main and 'strong'
dependencies between them. While these assumptions where presumably
correct in current code/data model context, they were logically wrong
and could have randomly cause bugs in the future.
* The sanitiing check on usercount performed in the case of the single
ID deletion cannot be done anymore. Should not be that usefull
anymore, as there are other places where IDs usercounts are validated.
* Performances:
* The batch-deletion did not show any significant difference in speed.
* Massively deleting IDs one by one however, showed a surprising 10%
speedup.
Pull Request: https://projects.blender.org/blender/blender/pulls/118283
f2e481a519