tests/subsys/shell: 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 <[email protected]>

Author: aescolar

tests/subsys/shell/shell_flash/testcase.yaml

@@ -8,5 +8,4 @@ tests:
       - shell
     platform_allow:
       - qemu_x86
-      - native_posix
       - native_sim