Revert timeline following in replication slots

This reverts commits f07d18b6e9, 82c83b3372, 3a3b309041, and
24c5f1a103.

This feature has shown enough immaturity that it was deemed better to
rip it out before rushing some more fixes at the last minute.  There are
discussions on larger changes in this area for the next release.

src/include/access/xlogreader.h

@@ -27,10 +27,6 @@
 
 #include "access/xlogrecord.h"
 
-#ifndef FRONTEND
-#include "nodes/pg_list.h"
-#endif
-
 typedef struct XLogReaderState XLogReaderState;
 
 /* Function type definition for the read_page callback */
@@ -164,25 +160,11 @@ struct XLogReaderState
 
 	/* beginning of the WAL record being read. */
 	XLogRecPtr	currRecPtr;
-	/* timeline to read it from, 0 if a lookup is required */
-	TimeLineID	currTLI;
-	/*
-	 * Safe point to read to in currTLI.  If currTLI is historical, then this
-	 * is set to the end of the last whole segment that contains that TLI;
-	 * if currTLI is ThisTimeLineID, this is InvalidXLogRecPtr.  This is *not*
-	 * the tliSwitchPoint.
-	 */
-	XLogRecPtr	currTLIValidUntil;
 
 	/* Buffer for current ReadRecord result (expandable) */
 	char	   *readRecordBuf;
 	uint32		readRecordBufSize;
 
-#ifndef FRONTEND
-	/* cached timeline history, only available in backend */
-	List	   *timelineHistory;
-#endif
-
 	/* Buffer to hold error message */
 	char	   *errormsg_buf;
 };