`proto_root` was appended to `dir_out` earlier, so don't duplicate it here

Signed-off-by: Alejandro R. Sedeño <asedeno@google.com>

bazel/generate_cc.bzl

@@ -134,7 +134,7 @@ def generate_cc_impl(ctx):
 
     # Include the output directory so that protoc puts the generated code in the
     # right directory.
-    arguments.append("--proto_path={0}{1}".format(dir_out, proto_root))
+    arguments.append("--proto_path={0}".format(dir_out))
     arguments += [_get_srcs_file_path(proto) for proto in protos]
 
     # create a list of well known proto files if the argument is non-None