Remove deb.tpo obfs4proxy Debian packages
obfs4proxy was package in Debian in #12910 (moved), but is currently only available in Debian testing and unstable.
Producing backports for Debian jessie and currently supported Ubuntu suites would improve availability to relay operators, and make it easier for relay operators to run useful bridges.
I'm happy to do this work, but it should be coordinated with infinity0/lunar who currently maintain the package in Debian. I'm also happy for them to do the backporting work.
For the Ubuntu backports, we should host these on deb.tpo (and if we choose to do so we should host Debian packages (sid, stretch, jessie) there too for completeness.