dsh: Move dsh related code into a module

Removed the dsh_groups define since that wasn't
used anywhere. It was introduced in
I0b86c6939821434ff40e5360e17d21f6e0a3ee3f with
a note saying 'not included anywhere', and it has
been more than a year and it still is not used anywhere.

Change-Id: Ic1bacaae8b3ef6fec2065ab8b63e72f3dcf6b2d5