Remove leftover dead code from commit_ts.h.

Commit 08aa89b326 removed the COMMIT_TS_SETTS WAL record,
leaving xl_commit_ts_set and SizeOfCommitTsSet unused. However,
it missed removing these definitions. This commit cleans up
the leftover code.

Since this is a cleanup rather than a bug fix, it is applied only to
the master branch.

Author: Andy Fan <zhihuifan1213@163.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/87ecuzmkqf.fsf@163.com

src/include/access/commit_ts.h

@@ -46,17 +46,6 @@ extern int	committssyncfiletag(const FileTag *ftag, char *path);
 #define COMMIT_TS_ZEROPAGE		0x00
 #define COMMIT_TS_TRUNCATE		0x10
 
-typedef struct xl_commit_ts_set
-{
-	TimestampTz timestamp;
-	RepOriginId nodeid;
-	TransactionId mainxid;
-	/* subxact Xids follow */
-}			xl_commit_ts_set;
-
-#define SizeOfCommitTsSet	(offsetof(xl_commit_ts_set, mainxid) + \
-							 sizeof(TransactionId))
-
 typedef struct xl_commit_ts_truncate
 {
 	int64		pageno;