Remove obsolete abstraction JobExecutor(Service)

Refer to Till's comment

The JobExecutor and the JobExecutorService have been introduced to bridge between the Flip-6 MiniCluster and the legacy FlinkMiniCluster. They should be obsolete now and could be removed if needed.

Actually we should make used of MiniClusterClient for submission ideally but we have some tests based on MiniCluster in flink-runtime or somewhere that doesn't have a dependency to flink-client; while move MiniClusterClient to flink-runtime is unclear whether reasonable or not. Thus I'd prefer keep executeJobBlocking for now and defer the possible refactor.