mtd: Remove redundant mutex from mtd_blkdevs.c

mtd: Remove redundant mutex from mtd_blkdevs.c

In commit 2a48fc0ab24241755dc93bfd4f01d68efab47f5a ('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
480792b7bf188c29b8d4b10fee65c3a06ec5dbf7 ('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>