Remove dead NoMovementScanDirection code

Here remove some dead code from heapgettup() and heapgettup_pagemode()
which was trying to support NoMovementScanDirection scans.  This code can
never be reached as standard_ExecutorRun() never calls ExecutePlan with
NoMovementScanDirection.

Additionally, plans which were scanning an unordered index would use
NoMovementScanDirection rather than ForwardScanDirection.  There was no
real need for this, so here we adjust this so we use ForwardScanDirection
for unordered index scans.  A comment in pathnodes.h claimed that
NoMovementScanDirection was used for PathKey reasons, but if that was
true, it no longer is, per code in build_index_paths().

This does change the non-text format of the EXPLAIN output so that
unordered index scans now have a "Forward" scan direction rather than
"NoMovement".  The text format of EXPLAIN has not changed.

Author: Melanie Plageman
Reviewed-by: Tom Lane, David Rowley
Discussion: https://postgr.es/m/CAAKRu_bvkhka0CZQun28KTqhuUh5ZqY=_T8QEqZqOL02rpi2bw@mail.gmail.com

src/include/access/sdir.h

@@ -16,8 +16,10 @@
 
 
 /*
- * ScanDirection was an int8 for no apparent reason. I kept the original
- * values because I'm not sure if I'll break anything otherwise.  -ay 2/95
+ * Defines the direction for scanning a table or an index.  Scans are never
+ * invoked using NoMovementScanDirectionScans.  For convenience, we use the
+ * values -1 and 1 for backward and forward scans.  This allows us to perform
+ * a few mathematical tricks such as what is done in ScanDirectionCombine.
  */
 typedef enum ScanDirection
 {
@@ -26,6 +28,13 @@ typedef enum ScanDirection
 	ForwardScanDirection = 1
 } ScanDirection;
 
+/*
+ * Determine the net effect of two direction specifications.
+ * This relies on having ForwardScanDirection = +1, BackwardScanDirection = -1,
+ * and will probably not do what you want if applied to any other values.
+ */
+#define ScanDirectionCombine(a, b)  ((a) * (b))
+
 /*
  * ScanDirectionIsValid
  *		True iff scan direction is valid.