target-cris: Remove unnecessary ifdef from mmu.c

mmu.c is only built for CONFIG_SOFTMMU targets, so there is
no need to redundantly surround the whole file contents with
an #ifndef CONFIG_USER_ONLY. The ifdef also confuses the
Coccinelle tool.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>