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>

diff --cc drivers/mtd/ubi/fastmap-wl.c
Simple merge

diff --cc drivers/mtd/ubi/wl.c
Simple merge

diff --cc drivers/mtd/ubi/wl.h
Simple merge