greybus: connection: remove WARN_ON from destroy

Remove WARN_ON from connection destroy testing for a NULL-connection
when destroying a connection.

This will allow for simpler driver code when releasing driver-managed
connections.

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