block: remove bdrv_dirty_bitmap_make_anon

block: remove bdrv_dirty_bitmap_make_anon

All this function is doing will be repeated by
bdrv_do_release_matching_dirty_bitmap_locked, except
resetting bm->persistent.  But even that does not matter
because the bitmap will be freed.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20180323164254.26487-1-pbonzini@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>