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

Author: tglsfdc

doc/src/sgml/syntax.sgml

@@ -1697,9 +1697,6 @@ SELECT string_agg(a ORDER BY a, ',') FROM table; -- incorrect
  </para>
 
  <para>
- <indexterm>
- <primary>median</primary>
- </indexterm>
  <indexterm>
  <primary>median</primary>
  <seealso>percentile</seealso>