Remove old ExecutorPlugin API (or wrap it using new API)

The parent bug is proposing a new plugin API for Spark, so we could remove the old one since it's a developer API.

If we can get the new API into Spark 3.0, then removal might be a better idea than deprecation. That would be my preference since then we can remove the new elements added as part of SPARK-28091 without having to deprecate them first.

If it doesn't make it into 3.0, then we should deprecate the APIs from 3.0 and manage old plugins through the new code being added.