Fix another outdated comment.

Preloading is done by logtape.c now.

src/backend/utils/sort/tuplesort.c

@@ -2882,9 +2882,8 @@ mergeonerun(Tuplesortstate *state)
  * beginmerge - initialize for a merge pass
  *
  * We decrease the counts of real and dummy runs for each tape, and mark
- * which tapes contain active input runs in mergeactive[].  Then, load
- * as many tuples as we can from each active input tape, and finally
- * fill the merge heap with the first tuple from each active tape.
+ * which tapes contain active input runs in mergeactive[].  Then, fill the
+ * merge heap with the first tuple from each active tape.
  */
 static void
 beginmerge(Tuplesortstate *state)