mtd: maps: ixp4xx: Drop driver

This custom maps driver was never used with the device tree
enabled IXP4xx systems, which use the physmap add-on from
commit 2aba2f2a
("mtd: physmap_of: add a hook for Intel IXP4xx flash probing")

Since kernel v5.18-rc1 IXP4xx is only booting from the device
tree so drop this old driver.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220407210844.2489682-1-linus.walleij@linaro.org