base-modem: fix modem_sync() operation handling

The mm_base_modem_sync() method is an asynchronous method that
receives a callback and user data, and therefore we MUST always
complete the async method calling that callback. Set that up with a
GTask as usual.

Also, the mm_base_modem_sync_finish() method should be implemented
along with mm_base_modem_sync(), not in the source file of the
caller of the async method. The finish() always depends on how the
async method was implemented, in our case using a GTask.

src/mm-base-manager.c

@@ -729,13 +729,6 @@ mm_base_manager_num_modems (MMBaseManager *self)
 
 #if defined WITH_SYSTEMD_SUSPEND_RESUME
 
-gboolean mm_base_modem_sync_finish (MMBaseModem   *self,
-                                    GAsyncResult  *res,
-                                    GError       **error)
-{
-    return g_task_propagate_boolean (G_TASK (res), error);
-}
-
 static void
 base_modem_sync_ready (MMBaseModem  *self,
                        GAsyncResult *res,