[ARM] 4558/1: pxa: remove MACH_TYPE_LUBBOCK assignment and leave it to boot loader

since both u-boot and blob support passing MACH_TYPE_LUBBOCK to the
kernel, it should be quite safe to remove this

Signed-off-by: eric miao <eric.y.miao@gmail.com>
Acked-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>