Remove jdbc stats collection mechanism

Though there are some deployments using it, usually its painful to setup since a valid hive-site.xml is needed on all task nodes (containing connection details) and for large tasks (with thousands of tasks) results in a scalability issue with all of them hammering DB at nearly same time.
Because of these pain points alternative stats collection mechanism were added. FS stats based system is default for some time.
We should remove jdbc stats collection mechanism as it needlessly adds complexity in TS and FS operators w.r.t key handling.