remove unused dependency protobuf3-to-dict

No occurrence in the source code:
```shell
> ag protobuf_to_dict
doc/frameworks/tensorflow/upgrade_from_legacy.rst
230:    from protobuf_to_dict import protobuf_to_dict
240:    protobuf_to_dict(json_format.Parse(result, tensor_pb2.TensorProto()))
```

This dependency has not been updated since 2017 and causes issues in certain combinations of poetry and pyenv. I suggest to remove it.

Author: zenogantner

setup.py

@@ -52,7 +52,6 @@ def read_requirements(filename):
     "google-pasta",
     "numpy>=1.9.0,<2.0",
     "protobuf>=3.1,<4.0",
-    "protobuf3-to-dict>=0.1.5,<1.0",
     "smdebug_rulesconfig==1.0.1",
     "importlib-metadata>=1.4.0,<5.0",
     "packaging>=20.0",