Simplify and optimize remapping handling in bacth ID deletion, by moving
it outside of the initial loop gathering all IDs to be deleted, and and
by using batch remapping code.
Speedup can be over 100 times faster in complex production scenes using
thousands of IDs, when e.g. deleting a whole library.
Examples before/after times on my machine (deleting two different libraries):
lib1: 5.55 sec/0.03 sec
lib2: 13.60 sec/0.13 sec
Found while investigating T101903.
358155a8da