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-role-cork.c

@@ -311,7 +311,7 @@ void pa__done(pa_module *m) {
         pa_hook_slot_free(u->sink_input_move_finish_slot);
 
     if (u->cork_state)
-        pa_hashmap_free(u->cork_state, NULL, NULL);
+        pa_hashmap_free(u->cork_state, NULL);
 
     pa_xfree(u);