Remove SchedulerExtensionService

This was added in SPARK-11314, but it has a few issues:

• it's in the YARN module, which is not a public Spark API.
• because of that it's also YARN specific
• it's not used by Spark in any way other than providing this as an extension point

For the latter, it probably makes sense to use listeners instead, and enhance the listener interface if it's lacking.

Also pinging steve_l since he added that code.