hashmap: Use pa_free_cb_t instead of pa_free2_cb_t

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.