mtd: nand: remove doubled chip deselection on (un)lock

Chip deselection is already done in nand_release_device. So only
duplicate code.

Signed-off-by: Jiri Pinkava <jiri.pinkava@vscht.cz>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>