ARM: imx: drop 'ARCH_MX31' and 'ARCH_MX35'

Commit 27ad4bf7 ("ARM: imx: move mx3 support to mach-imx") kept
ARCH_MX31 and ARCH_MX35 'for compatibility'. Now that they aren't
actually used anymore, they can be dropped entirely.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>