Remove bogus tracepoint

Calls to LWLockWaitForVar() fired the TRACE_POSTGRESQL_LWLOCK_ACQUIRE
tracepoint, but LWLockWaitForVar() never actually acquires the LWLock.
(Probably a copy/paste bug in 68a2e52b.)  Remove it.

Author: Craig Ringer <craig.ringer@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/CAGRY4nxJo+-HCC2i5H93ttSZ4gZO-FSddCwvkb-qAfQ1zdXd1w@mail.gmail.com

src/backend/storage/lmgr/lwlock.c

@@ -1725,8 +1725,6 @@ LWLockWaitForVar(LWLock *lock, uint64 *valptr, uint64 oldval, uint64 *newval)
 		/* Now loop back and check the status of the lock again. */
 	}
 
-	TRACE_POSTGRESQL_LWLOCK_ACQUIRE(T_NAME(lock), LW_EXCLUSIVE);
-
 	/*
 	 * Fix the process wait semaphore's count for any absorbed wakeups.
 	 */