Remove unused variable

The low variable has not been used since it was added in d168b66
and can be safely removed.  The variable is present in the Sedgewick
paper "Analysis of Shellsort and Related Algorithms" as a parameter
to the shellsort function, but our implementation does not use it.
Remove to improve readability of the code.

Author: Koki Nakamura <btnakamurakoukil@oss.nttdata.com>
Discussion: https://postgr.es/m/8aeb7b3eda53ca4c65fbacf8f43628fb@oss.nttdata.com

Author: danielgustafsson

src/backend/access/heap/heapam.c

@@ -8302,7 +8302,6 @@ index_delete_sort(TM_IndexDeleteOp *delstate)
 {
 	TM_IndexDelete *deltids = delstate->deltids;
 	int			ndeltids = delstate->ndeltids;
-	int			low = 0;
 
 	/*
 	 * Shellsort gap sequence (taken from Sedgewick-Incerpi paper).
@@ -8318,7 +8317,7 @@ index_delete_sort(TM_IndexDeleteOp *delstate)
 
 	for (int g = 0; g < lengthof(gaps); g++)
 	{
-		for (int hi = gaps[g], i = low + hi; i < ndeltids; i++)
+		for (int hi = gaps[g], i = hi; i < ndeltids; i++)
 		{
 			TM_IndexDelete d = deltids[i];
 			int			j = i;