vacuumlazy.c: Remove obsolete 'onecall' comment.

Remove obsolete reference to lazy_vacuum()'s onecall argument.  The
function argument was removed by commit 3499df0dee.

Also remove adjoining comment block that introduces the wraparound
failsafe concept.  Talking about the failsafe here no longer makes
sense, since lazy_vacuum() (and related functions) are no longer the
only place where the failsafe might be triggered.  This has been the
case since commit c242baa4a8 taught VACUUM to consider triggering the
failsafe mechanism during its initial heap scan.

src/backend/access/heap/vacuumlazy.c

@@ -2073,13 +2073,7 @@ retry:
  *
  * We may choose to bypass index vacuuming at this point, though only when the
  * ongoing VACUUM operation will definitely only have one index scan/round of
- * index vacuuming.  Caller indicates whether or not this is such a VACUUM
- * operation using 'onecall' argument.
- *
- * In rare emergencies, the ongoing VACUUM operation can be made to skip both
- * index vacuuming and index cleanup at the point we're called.  This avoids
- * having the whole system refuse to allocate further XIDs/MultiXactIds due to
- * wraparound.
+ * index vacuuming.
  */
 static void
 lazy_vacuum(LVRelState *vacrel)