mtd: nand: remove meaningless delay from nand_unlock

This delay is meaningless. If delay is needed it is device specific
and must be reimplemented by specific driver, otherwise no delay is
needed.

Signed-off-by: Jiri Pinkava <jiri.pinkava@vscht.cz>
Acked-by: Vimal Singh <vimal.newwork@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>