mtd: maps: remove obsolete impa7 map driver

mtd: maps: remove obsolete impa7 map driver

This driver was originally merged in 2002 for a board using
the Cirrus Logic CL/PS711x platform, but the actual board
file never made it upstream.

The SoC platform is still supported but uses devicetree
based probing, so if anyone ever wanted to upstream board
support, they would just use the regular physmap driver.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>