[STREAMING] [DOC] Remove duplicated description about WAL

I noticed there is a duplicated description about WAL.

```
To ensure zero-data loss, you have to additionally enable Write Ahead Logs in Spark Streaming. To ensure zero data loss, enable the Write Ahead Logs (introduced in Spark 1.2).
```

Let's remove the duplication.

I don't file this issue in JIRA because it's minor.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #6719 from sarutak/remove-multiple-description and squashes the following commits:

cc9bb21 [Kousuke Saruta] Removed duplicated description about WAL