Remove stray unused variable from Set iterator (#26675)

[minor fix, not reviewed]

Paul pointed this out, it was a consequence of all the code evolution
that branch took.

Passed a fresh paratest with futures

modules/standard/Set.chpl

@@ -563,7 +563,6 @@ module Set {
 
     @chpldoc.nodoc
     iter const these(param tag) where tag == iterKind.leader {
-      var space = 0..#_htb.tableSize;
       for followThis in _htb._evenSlots(tag) {
         yield followThis;
       }