Bazel: Remove deprecated experimental_shortened_obj_file_path option

`experimental_shortened_obj_file_path` is depreacted since [version `0.17.0`](https://blog.bazel.build/2018/09/14/bazel-0.17.html) and activated by default. Since Tensorflow requires [at least bazel `0.19.0`](https://github.com/tensorflow/tensorflow/blob/master/configure.py#L1559), this flag can be savely removed.