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/sierra/mm-broadband-bearer-sierra.c

@@ -311,7 +311,7 @@ dial_3gpp_context_step (Dial3gppContext *ctx)
     case DIAL_3GPP_STEP_LAST:
         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/sierra/mm-broadband-modem-sierra-icera.c

@@ -66,7 +66,7 @@ broadband_bearer_sierra_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);
 }

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

@@ -1115,7 +1115,7 @@ broadband_bearer_sierra_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);
 }