This patch makes two changes:

* Remove unnecessary `set SESSION wsrep_sync_wait=0`.
  Disabling `wsrep_sync_wait` caused the test to fail occasionally,
  due to subsequent `SELECT`s observing stale values.

* Remove redundant `--source include/galera_wait_ready.inc`,
  `galera_wait_ready.inc` is already included in the above
  `wait_until_connected_again.inc`