[FrameworkBundle] HHVM built-in web server is now deprecated

#10017 has introduced support for the HHVM built-in web server.

This web-server server is now deprecated, see:
- https://github.com/facebook/hhvm/wiki/Libevent-Removal
- http://hhvm.com/blog/4349/hhvm-3-0-0

I think it is still in the sources but no more delivered with the packages. Not sure if it's worth keeping this ?
