CLEANUP: pattern: remove pat_delete_fcts[] and pattern_head->delete()

CLEANUP: pattern: remove pat_delete_fcts[] and pattern_head->delete()

These ones are not used anymore, so let's remove them to remove a bit
of the complexity. The ACL keyword's delete() function could be removed
as well, though most keyword declarations are positional and we have a
high risk of introducing a mistake here, so let's not touch the ACL part.