mtd: remove lart flash driver

The sa1100 lart platform was removed, so its flash driver is
no longer useful.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20221021155000.4108406-7-arnd@kernel.org