mtd: Remove redundant mutex from mtd_blkdevs.c

mtd: Remove redundant mutex from mtd_blkdevs.c

In commit f525a846c435ad400506edd5216e88b775c3e63f ('block: autoconvert
trivial BKL users to private mutex'), Arnd replaced the BKL usage with a
mutex. However, Maxim has already provided a better fix in commit
7e519f26d94811a32459c2319bd152602e5433d7 ('mtd: blktrans: kill BKL'),
which was simply to remove the BKL without replacing it — since he'd
already made it do all necessary locking for itself.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>