mtd: nand: Remove support for block locking/unlocking

Commit 7d70f334 ("mtd: nand: add lock/unlock routines") introduced
support for the Micron LOCK/UNLOCK commands but no one ever used the
nand_lock/unlock() functions.

Remove support for these vendor-specific operations from the core. If
one ever wants to add them back they should be put in nand_micron.c and
mtd->_lock/_unlock should be directly assigned from there instead of
exporting the functions.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>