tests/subsys/modem: 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/modem/backends/tty/testcase.yaml

@@ -6,7 +6,6 @@ tests:
     tags: modem_backend_tty
     harness: ztest
     platform_allow:
-      - native_posix
       - native_sim
     integration_platforms:
       - native_sim

tests/subsys/modem/modem_chat/testcase.yaml

@@ -6,7 +6,6 @@ tests:
     tags: modem_chat
     harness: ztest
     platform_allow:
-      - native_posix
       - native_sim
     integration_platforms:
       - native_sim

tests/subsys/modem/modem_cmux/testcase.yaml

@@ -6,7 +6,6 @@ tests:
     tags: modem_cmux
     harness: ztest
     platform_allow:
-      - native_posix
       - native_sim
     integration_platforms:
       - native_sim

tests/subsys/modem/modem_ppp/testcase.yaml

@@ -6,7 +6,6 @@ tests:
     tags: modem_ppp
     harness: ztest
     platform_allow:
-      - native_posix
       - native_sim
     integration_platforms:
       - native_sim