arch/posix: Remove support for CONFIG_NATIVE_APPLICATION

CONFIG_NATIVE_APPLICATION, which represents the way in which
native_posix and its kin were built, was deprecated in 4.1 after the
introduction of native_sim and CONFIG_NATIVE_LIBRARY.

We remove support for it now for 4.3.

Let's remove the architecture layer adaptation code and cmake support.

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