qtjsbackend: remove for git versions

Found in [1]:
Qt Qml is now using its own built-in Javascript engine and does not depend on
V8 anymore. As such the QtJSBackend shared library has disappeared.

[1] http://blog.qt.digia.com/blog/2013/09/30/qt-5-2-alpha-available/

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>