Remove old unused COPYING-DOCS file

See:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=738537#10
https://git.gnome.org/browse/nautilus/commit/?id=f933accf04863e3e0f5828e9ba7655861057c2bc

Closes #251
https://github.com/mate-desktop/caja/issues/251