[MTD] mtd_blkdevs.c: Remove DEVFS leftovers

Remove mtd_blkdevs refs to the no longer functional DEVFS filesystem.

Verified mtdblock continues to work fine via udev with these calls
removed.

Signed-off-by: Todd Poynor <tpoynor@mvista.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>