Remove "show source" functionality

• show_source_page.php: removed

• core/print_api.php
  (print_source_link): removed

• core/html_api.php
  (html_footer): remove call to print_source_link()

• config_defaults_inc.php
  (g_show_source): removed

• doc/configuration.html: removed section on g_show_source

• lang/strings_english.txt: removed strings for show_source_page.php

• core/obsolete.php: added entry for show_source

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@2072 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9