plugins: remove explicit GDestroyNotify cast on g_free / g_object_unref

g_free and g_object_unref are in form of  `void (*)(gpointer)`, which
matches the GDestroyNotify signature. An explicit GDestroyNotify cast on
g_free and g_object_unref is thus not needed.

plugins/mbm/mm-broadband-bearer-mbm.c

@@ -178,7 +178,7 @@ handle_e2nap_connect_status (Dial3gppContext *ctx)
         mm_dbg ("Connected status indicated already by an unsolicited message");
         g_simple_async_result_set_op_res_gpointer (ctx->result,
                                                    g_object_ref (ctx->data),
-                                                   (GDestroyNotify)g_object_unref);
+                                                   g_object_unref);
         dial_3gpp_context_complete_and_free (ctx);
         return TRUE;
     case MM_BEARER_CONNECTION_STATUS_DISCONNECTED:
@@ -220,7 +220,7 @@ connect_poll_ready (MMBaseModem *modem,
         /* Success!  Connected... */
         g_simple_async_result_set_op_res_gpointer (ctx->result,
                                                    g_object_ref (ctx->data),
-                                                   (GDestroyNotify)g_object_unref);
+                                                   g_object_unref);
         dial_3gpp_context_complete_and_free (ctx);
         return;
     }

plugins/mbm/mm-broadband-modem-mbm.c

@@ -111,7 +111,7 @@ broadband_bearer_mbm_new_ready (GObject *source,
     else
         g_simple_async_result_set_op_res_gpointer (simple,
                                                    bearer,
-                                                   (GDestroyNotify)g_object_unref);
+                                                   g_object_unref);
     g_simple_async_result_complete (simple);
     g_object_unref (simple);
 }