mtd: maps: remove dead MTD map driver for PMC-Sierra MSP boards

Commit 1b00767f ("MIPS: Remove PMC MSP71xx platform") removes the
config PMC_MSP in ./arch/mips/Kconfig.

Hence, since then, the corresponding MTD map driver for PMC-Sierra MSP
boards is dead code. Remove this dead driver.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210817104531.12675-1-lukas.bulwahn@gmail.com