greybus: gpbridge: Remove (now) unused macro gb_gpbridge_builtin_driver()

gb_gpbridge_builtin_driver() isn't used anymore, remove it.

Tested on EVT 1.5 by inserting GP test module, all the devices were
enumerated correctly.

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