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-