iface-modem: ignore cancellation during sync()

There is no input cancellable in the method, so the GTask will never
get cancelled from the outside.

src/mm-iface-modem.c

@@ -4276,12 +4276,6 @@ interface_syncing_step (GTask *task)
     MMIfaceModem   *self;
     SyncingContext *ctx;
 
-    /* Don't run new steps if we're cancelled */
-    if (g_task_return_error_if_cancelled (task)) {
-        g_object_unref (task);
-        return;
-    }
-
     self = g_task_get_source_object (task);
     ctx = g_task_get_task_data (task);