Doc: Remove MultiXact wraparound section link.

Doc: Remove MultiXact wraparound section link.

Remove circular "25.1.5.1. Multixacts And Wraparound" link that
references the section that the link itself appears in.  An explanation
of MultiXactId age appears only a few sentences before the link, so
there's no question that the link is superfluous at best.

Oversight in commit d5409295.

Author: Peter Geoghegan <pg@bowt.ie>
Backpatch: 14-