remote: drop code for migrating config files from pre-XDG dir layout

remote: drop code for migrating config files from pre-XDG dir layout

The unprivileged libvirtd daemon switched to use the XDG dir layout in
the 0.9.13 release, and included code for moving config files from the
old location. The chances of someone upgrading libvirt from <= 0.9.12
directly to libvirt >= 5.5.0 is close enough to zero that we can
reasonably drop the back compat code.

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>