All core tests compile without grpc_mdstr

grpc.def

include/grpc/grpc.h

include/grpc/slice.h

src/core/ext/transport/chttp2/transport/hpack_encoder.c

src/core/ext/transport/chttp2/transport/hpack_parser.c

src/core/lib/iomgr/ev_posix.c

...core/lib/security/credentials/plugin/plugin_credentials.c

src/core/lib/slice/slice.c

src/core/lib/slice/slice_intern.c

src/core/lib/slice/slice_internal.h

src/core/lib/slice/slice_string_helpers.c

src/core/lib/surface/call.c

src/core/lib/surface/validate_metadata.c

src/core/lib/transport/metadata.c

src/ruby/ext/grpc/rb_grpc_imports.generated.c

src/ruby/ext/grpc/rb_grpc_imports.generated.h

test/core/bad_client/tests/large_metadata.c

test/core/bad_client/tests/simple_request.c

test/core/bad_ssl/bad_ssl_test.c

test/core/channel/channel_args_test.c