Documentation: remove Twisted Matrix pre-req

Twisted Matrix is no longer necessary for Xen.  Removed from the
pre-requisites listed in the user documentation.

Signed-off-by: Andrew D. Ball <aball@us.ibm.com>

docs/src/user/installation.tex

@@ -24,11 +24,6 @@ required if you wish to build from source.
 \item [$\dag$] The Linux hotplug system\footnote{Available from {\tt
       http://linux-hotplug.sourceforge.net/}} (e.g., \path{/sbin/hotplug}
       and related scripts)
-\item [$\dag$] An installation of Twisted~v1.3 or
-  above\footnote{Available from {\tt http://www.twistedmatrix.com}}.
-  There may be a binary package available for your distribution;
-  alternatively it can be installed by running `{\sl make
-    install-twisted}' in the root of the Xen source tree.
 \item [$*$] Build tools (gcc v3.2.x or v3.3.x, binutils, GNU make).
 \item [$*$] Development installation of libcurl (e.g.,\ libcurl-devel).
 \item [$*$] Development installation of zlib (e.g.,\ zlib-dev).