mtd: rawnand: Stop using nand_release()

This helper is not very useful and very often people get confused:
they use nand_release() instead of nand_cleanup().

Now that all drivers have been converted to do not use nand_release()
anymore, let's remove this helper.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/linux-mtd/20200519130035.1883-63-miquel.raynal@bootlin.com