test suite: Drop all testing of old tag formats, and compatibility

test suite: Drop all testing of old tag formats, and compatibility

This is quite old now.  We are dropping support for it.

The new tags came in in dgit 2.0 in October 2016.  No version of dgit
without them was in any Debian release.  We don't expect there are any
sites using old servers which do not support the new format, or anyone
using a dgit client older than 2.x who cannot upgrade.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>