Remove Vagrantfile (will be moved into libevent-extras)

Since:
- it is not a library
- this file should have (if I had enough time) enough fixes in itself
  and should not polute libevent history
- it "requires" (it more cleaner to use it in this way) script --
  tools/vagrant-tests.py (indeed, from libevent-extras)
- will has it's own issues/README/...

https://github.com/libevent/libevent-extras