microblaze: Removing dead CONTIGUOUS_PAGE_ALLOC config option

CONFIG_CONTIGUOUS_PAGE_ALLOC doesn't exist in Kconfig, therefore removing
all references to it from the source.

Signed-off-by: Christian Dietrich <qy03fugy@stud.informatik.uni-erlangen.de>
Signed-off-by: Michal Simek <monstr@monstr.eu>