tests/subsys/mem_mgmt: Don't test on native_posix

native_posix is now deprecated.
Building these tests in both native_sim and native_posix does not
improve coverage for the tests or subsystem but doubles CI time.
As anyhow native_posix will be removed all together in
2 releases, let's remove it already for these.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

tests/subsys/mem_mgmt/mem_attr/testcase.yaml

@@ -1,7 +1,5 @@
 common:
   platform_allow:
-    - native_posix
-    - native_posix/native/64
     - native_sim
     - native_sim/native/64
   integration_platforms: