Remove C++17 standard option from BUILD.bazel

Removed C++ standard option from protoc-gen-grpc-web.

Author: sampajano

javascript/net/grpc/web/generator/BUILD.bazel

@@ -5,7 +5,6 @@ cc_binary(
     srcs = [
         "grpc_generator.cc",
     ],
-    copts = ["-std=c++17"],
     visibility = ["//visibility:public"],
     deps = [
         "@com_google_protobuf//:protoc_lib",