arch: stm32: Remove unsupported MPU options

Some code was developped around MPU support in ealy stage
of feature introduction. This code was gated under MPU_ENABLE
flag which does not exist and hence code has never been
enabled nor tested.
Since there has been futher MPU development since then, it seems
safer to simply remove that code and associated Kconfig symbols
to avoid that someone starts using it while it was not part of
MPU development track.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>