Code cleanup in the wake of recent LWLock refactoring.

Code cleanup in the wake of recent LWLock refactoring.

As of commit c1772ad9225641c921545b35c84ee478c326b95e, there's no
longer any way of requesting additional LWLocks in the main tranche,
so we don't need NumLWLocks() or LWLockAssign() any more.  Also,
some of the allocation counters that we had previously aren't needed
any more either.

Amit Kapila