remove internal_stub

BUILD

Makefile

build.json

include/grpc++/impl/internal_stub.h

src/compiler/cpp_generator.cc

src/cpp/client/internal_stub.cc

tools/doxygen/Doxyfile.c++

tools/doxygen/Doxyfile.c++.internal

tools/run_tests/sources_and_headers.json

vsprojects/grpc++/grpc++.vcxproj

vsprojects/grpc++/grpc++.vcxproj.filters

vsprojects/grpc++_unsecure/grpc++_unsecure.vcxproj

vsprojects/grpc++_unsecure/grpc++_unsecure.vcxproj.filters