message: [c++ grpc] Don't ignore dirty grpc submodule
  The grpc submodule itself now marks it's zlib submodule as "ignore =
  dirty", so we no longer have to do that for grpc just to ignore zlib.
diff: [submodule "thirdparty/grpc"]
  path = thirdparty/grpc
  url = https://github.com/grpc/grpc.git
  - # When using CMake to build, the zlib submodule inside of the grpc
  - # submodule ends up with a generated file that makes Git consider
  - # the submodule dirty.
  - ignore = dirty
  [submodule "thirdparty/rapidjson"]
  path = thirdparty/rapidjson
  