Remove duplicate index entry that is repeated a few lines above.

Our current index format for HTML output doesn't support fine-grained
anchor points for indexterm's.  This means that we can have each
indexterm only _once_ in each <sect1> section.  Until we have a better
way of indexing multiple occurences of the same indexterm in a single
sect1, remove the duplicate.

diff -r 48c1e2e24a00 -r 71607d9ec804 en_US.ISO8859-1/books/handbook/basics/chapter.xml
--- a/en_US.ISO8859-1/books/handbook/basics/chapter.xml	Sun Mar 10 15:31:07 2013 +0100
+++ b/en_US.ISO8859-1/books/handbook/basics/chapter.xml	Sun Mar 10 18:32:20 2013 +0100
@@ -2146,22 +2146,20 @@ 198  inetd -wW</screen>
 
     <indexterm><primary>environment variables</primary></indexterm>
 
     <para>Another feature of the shell is the use of environment
       variables.  Environment variables are a variable/key pair stored
       in the shell's environment.  This environment can be read by any
       program invoked by the shell, and thus contains a lot of program
       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>
 	  <row>
 	    <entry>Variable</entry>
 	    <entry>Description</entry>
 	  </row>
 	</thead>
 
 	<tbody>