protoc-gen-grpc-java isn't useful in a BOM because the Gradle plugin
doesn't support BOMs (google/protobuf-gradle-plugin#412) and it seems
unlikely the Maven plugin would as well.
Right now the type is pom, which is useless; nobody cares about the pom
itself. We'd need to use a type of exe and repeat it for each platform
(classifier). Given that it is a strange case and we can't actually test
the results and having it in the bom might lead a user to trying to make
it work, let's just remove it for now until it provides value.
Fixes#8936