This fix removes deprecated support for config `SPARK_YARN_USER_ENV`, as is mentioned in SPARK-17979.
This fix also removes deprecated support for the following:
```
SPARK_YARN_USER_ENV
SPARK_JAVA_OPTS
SPARK_CLASSPATH
SPARK_WORKER_INSTANCES
```

Related JIRA:
[SPARK-14453]: https://issues.apache.org/jira/browse/SPARK-14453
[SPARK-12344]: https://issues.apache.org/jira/browse/SPARK-12344
[SPARK-15781]: https://issues.apache.org/jira/browse/SPARK-15781

Existing tests should pass.

Author: Yong Tang <yong.tang.github@outlook.com>

Closes #17212 from yongtang/SPARK-17979.