dgit-sponsorship(7): Use --no-dep14tag

While this complicates the sponsor's workflow a little, it simplifies
the sponsee's workflow: they don't have to decide whether to delete
DEP-14 tags that other guides have told them to create.

Closes:#849105.

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