Remove dead code supporting mark/restore in SeqScan, TidScan, ValuesScan.

There seems no prospect that any of this will ever be useful, and indeed
it's questionable whether some of it would work if it ever got called;
it's certainly not been exercised in a very long time, if ever. So let's
get rid of it, and make the comments about mark/restore in execAmi.c less
wishy-washy.

The mark/restore support for Result nodes is also currently dead code,
but that's due to planner limitations not because it's impossible that
it could be useful.  So I left it in.

src/include/access/heapam.h

@@ -161,9 +161,6 @@ extern void simple_heap_delete(Relation relation, ItemPointer tid);
 extern void simple_heap_update(Relation relation, ItemPointer otid,
 				   HeapTuple tup);
 
-extern void heap_markpos(HeapScanDesc scan);
-extern void heap_restrpos(HeapScanDesc scan);
-
 extern void heap_sync(Relation relation);
 
 /* in heap/pruneheap.c */

src/include/access/relscan.h

@@ -48,11 +48,9 @@ typedef struct HeapScanDescData
 	BlockNumber rs_cblock;		/* current block # in scan, if any */
 	Buffer		rs_cbuf;		/* current buffer in scan, if any */
 	/* NB: if rs_cbuf is not InvalidBuffer, we hold a pin on that buffer */
-	ItemPointerData rs_mctid;	/* marked scan position, if any */
 
 	/* these fields only used in page-at-a-time mode and for bitmap scans */
 	int			rs_cindex;		/* current tuple's index in vistuples */
-	int			rs_mindex;		/* marked tuple's saved index */
 	int			rs_ntuples;		/* number of visible tuples on page */
 	OffsetNumber rs_vistuples[MaxHeapTuplesPerPage];	/* their offsets */
 }	HeapScanDescData;