psgi: Remove the check for the async mode in uwsgi_perl_atexit

According to Roberto this is just copy/pasted from the Python code, see
1.0.1-289-gd7e8523.
psgi: Don't free the wsgi_req->async_environ atexit, only in request context

The atexit hooks can be called outside of the request context. See the
discussion at https://github.com/unbit/uwsgi/pull/772