posix: mem: remove imply MMU from POSIX options

With MMU being converted to a non-user-configurable option, we can
rely on that exclusively to know whether arch_mem_map() and
arch_mem_unmap() are available and we can remove the workarounds
in POSIX at the Kconfig level.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>