Doc: remove duplicate index entry.

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