greybus: interface: remove defensive WARN_ON

Remove defensive WARN_ON testing for a NULL-interface when removing an
interface.

Every call site has just dereferenced the interface.

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>