UBI: Fastmap: Remove is_fm_block()

This function was added to fastmap in a very early stage
to have paranoid assertions.
With the current fastmap implementation this assert will never
trigger as fastmap PEBs are not seen by the WL sub-system.
Remove it to save us some CPU cycles.

Signed-off-by: Richard Weinberger <richard@nod.at>
Git-commit: 56794c0a1ae738cb4184ab27501e4d152b0b2771
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>