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>