Blender Git Loki

Get rid of 'drivers unlinking' code in `BKE_libblock_free_data()`

This has nothing to do here (freeing is not unlinking/remapping!), and
was actually redoing something already taken care of by
`BKE_libblock_relink_ex()` call in `BKE_libblock_free_ex()`.

Also, gives some noticeable speedup when removing datablocks with
do_unlink=True, about 5 to 10% quicker e.g. when deleting all objects
from a py console, in a big production file...