greybus: legacy: remove protocol.o from the makefile

Commit 0917cba11 ("legacy: remove legacy driver support")
removed protocol.c, however, the corresponding target in the Makefile
was not removed therefore broken the build.

Testing Done:
- Build & boot on EVT1.5

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

drivers/staging/greybus/Makefile

@@ -6,7 +6,6 @@ greybus-y :=	core.o		\
 		interface.o	\
 		bundle.o	\
 		connection.o	\
-		protocol.o	\
 		control.o	\
 		svc.o		\
 		svc_watchdog.o	\