hashmap: Use pa_free_cb_t instead of pa_free2_cb_t

The previous patch removed module-gconf's dependency on the userdata
pointer of the free callback, and that was the only place where the
userdata pointer of pa_free2_cb_t was used, so now there's no need for
pa_free2_cb_t in pa_hashmap_free(). Using pa_free_cb_t instead allows
removing a significant amount of repetitive code.

src/modules/module-zeroconf-discover.c

@@ -426,7 +426,7 @@ void pa__done(pa_module*m) {
             tunnel_free(t);
         }
 
-        pa_hashmap_free(u->tunnels, NULL, NULL);
+        pa_hashmap_free(u->tunnels, NULL);
     }
 
     pa_xfree(u);