Cleanup: Remove unused SYNC_REC_LOCK

SYNC_REC_LOCK was never used in the public history of InnoDB,
starting with commit 132e667b.

storage/innobase/include/sync0types.h

@@ -227,7 +227,6 @@ enum latch_level_t {
 	SYNC_PAGE_CLEANER,
 	SYNC_PURGE_QUEUE,
 	SYNC_TRX_SYS_HEADER,
-	SYNC_REC_LOCK,
 	SYNC_THREADS,
 	SYNC_TRX,
 	SYNC_TRX_SYS,

storage/innobase/sync/sync0debug.cc

@@ -472,7 +472,6 @@ LatchDebug::LatchDebug()
 	LEVEL_MAP_INSERT(SYNC_PAGE_CLEANER);
 	LEVEL_MAP_INSERT(SYNC_PURGE_QUEUE);
 	LEVEL_MAP_INSERT(SYNC_TRX_SYS_HEADER);
-	LEVEL_MAP_INSERT(SYNC_REC_LOCK);
 	LEVEL_MAP_INSERT(SYNC_THREADS);
 	LEVEL_MAP_INSERT(SYNC_TRX);
 	LEVEL_MAP_INSERT(SYNC_TRX_SYS);
@@ -843,15 +842,6 @@ LatchDebug::check_order(
 		}
 		break;
 
-	case SYNC_REC_LOCK:
-
-		if (find(latches, SYNC_LOCK_SYS) != 0) {
-			basic_check(latches, level, SYNC_REC_LOCK - 1);
-		} else {
-			basic_check(latches, level, SYNC_REC_LOCK);
-		}
-		break;
-
 	case SYNC_IBUF_BITMAP:
 
 		/* Either the thread must own the master mutex to all