Doc: Remove reference to QtJsBackend

qtjsbackend.git was replaced by the new V4 JavaScript engine.

Change-Id: I8d305b9029ab54d1e5ec7e661a960757b10ed4d8
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>

doc/src/platforms/windows.qdoc

@@ -173,7 +173,7 @@
            the installation location to your \c PATH.
         \li Python - Install Python from the \l{http://www.python.org/download/}{here}
            and add the installation location to your PATH in order to be able
-           to build QtJsBackend and Qt WebKit.
+           to build Qt WebKit.
 
         \li Install \l{http://www.ruby-lang.org}{Ruby} from \l{http://rubyinstaller.org/}{here}
            and add the installation location to your PATH in order to be able