Fix inconsistencies and typos in the tree, take 10

This addresses some issues with unnecessary code comments, fixes various
typos in docs and comments, and removes some orphaned structures and
definitions.

Author: Alexander Lakhin
Discussion: https://postgr.es/m/9aabc775-5494-b372-8bcb-4dfc0bd37c68@gmail.com

src/backend/access/transam/slru.c

@@ -1364,7 +1364,7 @@ SlruScanDirCbDeleteAll(SlruCtl ctl, char *filename, int segpage, void *data)
 }
 
 /*
- * Scan the SimpleLRU directory and apply a callback to each file found in it.
+ * Scan the SimpleLru directory and apply a callback to each file found in it.
  *
  * If the callback returns true, the scan is stopped.  The last return value
  * from the callback is returned.

src/backend/access/transam/xact.c

@@ -1232,7 +1232,7 @@ RecordTransactionCommit(void)
 	if (!markXidCommitted)
 	{
 		/*
-		 * We expect that every smgrscheduleunlink is followed by a catalog
+		 * We expect that every RelationDropStorage is followed by a catalog
 		 * update, and hence XID assignment, so we shouldn't get here with any
 		 * pending deletes.  Use a real test not just an Assert to check this,
 		 * since it's a bit fragile.

src/backend/access/transam/xlogreader.c

@@ -702,7 +702,7 @@ ValidXLogRecordHeader(XLogReaderState *state, XLogRecPtr RecPtr,
  * We assume all of the record (that is, xl_tot_len bytes) has been read
  * into memory at *record.  Also, ValidXLogRecordHeader() has accepted the
  * record's header, which means in particular that xl_tot_len is at least
- * SizeOfXlogRecord.
+ * SizeOfXLogRecord.
  */
 static bool
 ValidXLogRecord(XLogReaderState *state, XLogRecord *record, XLogRecPtr recptr)