Consider dropping 'srcs' from java_grpc_library Bazel rule

This is to simplify java_grpc_library by only specifying the 'deps' pointing to the java_proto_library.

This has come up a few times internally as well as in #5237.

We'd need some cross-language agreement. In general we don't have many spare cycles to work on something like this cross-language right now, so there will probably be some push-back simply because what we have now "works."

CC @ceason