greybus: es2: remove svc endpoint message handling

We have switched over to use the "new" svc messages, no more need to
have a special USB endpoint to handle them, they come through the normal
CPort messages.

Based on a patch from Viresh.

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