Remove unnecessary assignment.

Reported by MauMau.

Author: hlinnaka

src/backend/storage/lmgr/lwlock.c

@@ -997,8 +997,6 @@ LWLockWaitForVar(LWLock *lock, uint64 *valptr, uint64 oldval, uint64 *newval)
 		 */
 		proc->lwWaiting = true;
 		proc->lwWaitMode = LW_WAIT_UNTIL_FREE;
-		proc->lwWaitLink = NULL;
-
 		/* waiters are added to the front of the queue */
 		proc->lwWaitLink = lock->head;
 		if (lock->head == NULL)