Remove obsolete helpers in flidas

(I couldn't select version 8.0, but this is for 8.0)

We need to clean up the package helpers. Some are obsolete because a licensing issue was fixed upstream, package no longer in repo, etc.

A few obsolete helpers are:

• cabal-install-1.20: flidas has a newer version. Someone might want to backport ghc 8, and I think that would be better placed in backports.
• ghc-7.8.3: Same as above.
• grace: Fixed upstream. The included copy of cephes was relicensed under the GPL with permission from the author, and the new license is there in xenial.
• sane-backends: Fixed upstream. snprintf.c was replaced with the free version. (However, this is not the case for the other sane packages that have helpers)
• tcltrf: Fixed upstream. Debian (and Ubuntu) have a free implementation of RIPEMD.
• unrar-free: Fixed upstream. unrar-free was updated to utilize unar if it encounters a RARv3 archive. It now recommends unar instead of the nonfree unrar.
• xchat: Package no longer exists. (However, a helper might need to be made for the fork hexchat.)

If you find more, please post them below.