dgit clone: No longer create an "origin" remote

dgit clone: No longer create an "origin" remote

This was a normal git remote pointing to the dgit git server.

This is not really sensible because
1. it can't simply be pushed to
2. sometimes (if the package was never pushed with dgit)
    it doesn't even exist
3. it may be out of date with respect to the archive

The effect of 2 is that it can break `git remote update'.

This "git remote add" was introduced in 2013 in
  39c6c123ab730d42ec7c9ed01c30e0175c7691e7
  fix branch usage
and was first included in 0.2.  There doesn't seem to be any evidence
of me having deeply considered the merits of this at the time.

Closes: #932694
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>