Remove obsolete (unreferenced) files

Signed-off-by: Roger Sheen <roger@infotexture.net>

resources/source-files.ditamap

@@ -146,7 +146,6 @@
   <keydef keys="release-notes" href="../release-notes/index.dita"/>
   <keydef keys="third-party-software" href="../reference/third-party-software.dita"/>
   <keydef keys="troubleshooting-overview" href="../topics/troubleshooting-overview.dita"/>
-  <keydef keys="user-guide" href="../topics/index.dita"/>
   <keydef keys="using-dita-command" href="../topics/using-dita-command.dita"/>
   <keydef keys="using-dita-properties-file" href="../topics/using-dita-properties-file.dita"/>
   <keydef keys="using-plugin-URI-extension" href="../reference/using-plugin-URI-extension.dita"/>