Drop the redundant "Lock" suffix from LWLock wait event names.

This was mostly confusing, especially since some wait events in
this class had the suffix and some did not.

While at it, stop exposing MainLWLockNames[] as a globally visible
name; any code using that directly is almost certainly wrong, as
its name has been misleading for some time.
(GetLWLockIdentifier() is what to use instead.)

Discussion: https://postgr.es/m/28683.1589405363@sss.pgh.pa.us

src/include/storage/lwlock.h

@@ -88,7 +88,6 @@ typedef union LWLockMinimallyPadded
 } LWLockMinimallyPadded;
 
 extern PGDLLIMPORT LWLockPadded *MainLWLockArray;
-extern const char *const MainLWLockNames[];
 
 /* struct for storing named tranche information */
 typedef struct NamedLWLockTranche