misc: assert_post_action: Treat POSIX arch like others

After #12732, 69045011
asserts call k_panic.

Before this, the POSIX arch had its own hack in the
__ASSERT_POST implementation to terminate the process instead
of spining forever.

But the POSIX arch does implement k_panic properly, so there
is no need anymore for this hack.
=> Remove the special treatment for POSIX ARCH

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>