Removed unused OtherFailure TaskEndReason.

The OtherFailure TaskEndReason was added by @mateiz 3 years ago in this commit: https://github.com/apache/incubator-spark/commit/24a1e7f8380bfd8d4fbdda688482a451bd6ea215

Unless I am missing something, it doesn't seem to have been used then, and is not used now, so seems safe for deletion.