Remove unnecessary line in MANIFEST.in for docs/avatar.png.

* The buildbot for ooni-backend was showing:
   Downloading/unpacking txtorcon>=0.7 (from -r .requirements.travis (line 21))
   Downloading txtorcon-0.7.tar.gz (128kB): 128kB downloaded
   Running setup.py egg_info for package txtorcon
   warning: no files found matching 'docs/avatar.png'