Remove the debian directory from the main git repository

Once upon a time it made sense to keep all the Debian files in the
main Tor distribution, since repeatedly merging them back in was hard.
Now that we're on git, that's no longer so.

Peter's debian repository at debian/tor.git on our git server has the
most recent version of the tor-on-debian packaging stuff, and the versions
in our own repository have gotten out of date.

Resolves bug #1735.