Remove dead code from pop_next_work_item().
Remove dead code from pop_next_work_item().

The pref_non_data heuristic has been dead code for nearly ten years,
and as far as I can tell was dead code even when it was first committed.
I'm tired of silencing Coverity complaints about it, so get rid of it.
If anyone is ever interested in pursuing the concept, they can get the
code out of our git history.

lisakowen/gpdb,xinzweb/gpdb,adam8157/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,lisakowen/gpdb,50wu/gpdb,adam8157/gpdb,adam8157/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,greenplum-db/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,lisakowen/gpdb