lib: delete lib/find_last_bit.c

The file lib/find_last_bit.c was no longer used and supposed to be
deleted by commit 8f6f19dd

 ("lib: move find_last_bit to
lib/find_next_bit.c") but that delete didn't happen.  This gets rid of
it.

Signed-off-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>