greybus: connection: kill gb_hd_connections_exit

Connections are destroyed as part of interface tear down. If we fail to
do that properly it's a bug that should be fixed rather than papered
over by a fall-back clean up function.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>