Remove ranch_listener and replace ListenerPid by Ref

We just don't need this process anymore. Less, simpler code!

API changes:

 *  Protocols start_link first argument is now Ref instead of ListenerPid
 *  ranch:accept_ack/1 argument is now Ref instead of ListenerPid
 *  ranch_listener:remove_connection/1 becomes ranch:remove_connection/1
    and its argument is now Ref instead of ListenerPid

Ref is the name of the listener given as first argument to start_listener/6.