19507: cpu/cc26x0_cc13x0: Drop feature cortexm_mpu r=maribu a=maribu
### Contribution description
At least the CC2650 doesn't have an MPU, I assume this is also true for the rest of the family.
The CC2652 does have an MPU according to the datasheet. So I keep the feature there in place.
### Testing procedure
E.g.
```
make BOARD=cc2650-launchpad -C tests/mpu_noexec_ram flash test
```
fails. (Note: A successful test run would also crash but with a mem manage handler rather than a hardfault due to an invalid instruction on the stack being executed.)
It would be nice to also test the same for a `cc2652-launchpad`, for which the MPU should work.
### Issues/PRs references
None
Co-authored-by: Marian Buschsieweke <marian.buschsieweke@ovgu.de>
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