This PR cldean up remaining left over of github.com/golang/protobuf in coredns,
as github.com/golang/protobuf has been deprecated.

Note the github.com/golang/protobuf is now only indirectly
used by external libraries. Manually updating coredns' dependency to futher clean up has been tried.
However, it was not possible as external library usages of github.com/golang/protobuf are too interleaved.

This PR fixes 4939.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>