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/pulsecore/mutex-win32.c

@@ -80,7 +80,7 @@ pa_cond *pa_cond_new(void) {
 void pa_cond_free(pa_cond *c) {
     assert(c);
 
-    pa_hashmap_free(c->wait_events, NULL, NULL);
+    pa_hashmap_free(c->wait_events, NULL);
     pa_xfree(c);
 }