Remove duplicate index entry, which is repeated a few lines above

We don't have support for multiple <indexterm> elements pointing to
different parts of the same <sect1> right now. It requires a few
radical changes to collateindex.pl, so we can only have _one_ point
for each indexterm in each sect1.

Approved by:	gjb (mentor)

en_US.ISO8859-1/books/handbook/basics/chapter.xml

@@ -2153,8 +2153,6 @@ Swap: 256M Total, 38M Used, 217M Free, 15% Inuse
       configuration.  Here is a list of common environment variables
       and their meanings:</para>
 
-    <indexterm><primary>environment variables</primary></indexterm>
-
     <informaltable frame="none" pgwide="1">
       <tgroup cols="2">
 	<thead>