Doc: remove duplicate index entry.

This causes a warning with the old html-docs toolchain, though not with the
new.  I had originally supposed that we needed both <indexterm> entries to
get both a primary index entry and a see-also link; but evidently not,
as pointed out by Fabien Coelho.

Discussion: https://postgr.es/m/alpine.DEB.2.20.1702161616060.5445@lancre

doc/src/sgml/syntax.sgml

@@ -1707,9 +1707,6 @@ SELECT string_agg(a ORDER BY a, ',') FROM table;  -- incorrect
    </para>
 
    <para>
-    <indexterm>
-     <primary>median</primary>
-    </indexterm>
     <indexterm>
      <primary>median</primary>
      <seealso>percentile</seealso>