microblaze: Remove unused NO_MMU Kconfig parameter

This removes the NO_MMU Kconfig parameter,
which was no longer used anywhere in the source code
and Makefiles.

This also updates a comment refering to this parameter.

Signed-off-by: Michael Opdenacker <michael.opdenacker@free-electrons.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>