Removing the attic package

[Leo Famulari]

The Attic backup program [0] has a serious problem, and I think we
should consider removing our package of it.

The problem is that Attic appears to be unmaintained since it was forked
as "Borg". For almost 11 months, there has been no response from the
Attic maintainer to a bug that unrecoverably corrupts the backup
repository when the disk becomes full [1]. Of course, this situation is
not acceptable for "backup" software.

Debian has stopped packaging Attic in response [2].

At the very least, I think we should add a strong warning to the package
description and synopsis.

[0]
https://attic-backup.org/

[1]
https://github.com/jborg/attic/issues/356

[2]
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=802619