[caffe2] Delete some unused fields from TensorProto (#52521)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52521
The `storage_type` and `external_data` fields were added a few years ago in
D10246743 (https://github.com/pytorch/pytorch/commit/30aaa075940cd2739f9786ee96c42b0caefd8094) but don't appear to have been used anywhere. Let's remove them to
help simplify the `TensorProto` message definition.
ghstack-source-id: 122110201
Test Plan: Confirmed the code still builds.
Reviewed By: dzhulgakov
Differential Revision: D26500028
fbshipit-source-id: 1e188f98f59e0b8673ea342ad9aaf7e5ba9b5fac
