Bug#920482: apt update fails to update from repository missing optional Contents files

[Andrew Worsley <amworsley@gmail.com>]

Now fixed by purging the apt-file package!
Big thanks to pabs on IRC #debian-apt for this solution.

Note: You need to purge the package to make sure the config file
apt-file supplies is removed to fix this problem (remove is *NOT*
enough)

Apparently apt-file no works by using apt-get to install the Contents
target for that file.
This can be configure as described in
/usr/share/doc/apt-file/README.md.gz for certain repositories via a
line like:
[target-=Contents-deb] $MIRROR/debian unstable main ...