[PyTorch] Remove unused function in import (#65865)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65865
`operator_str` is not used in `import.cpp` and it is also defined in `parse_operators.cpp` so removing it from `import.cpp`.
Test Plan: CI passing
Reviewed By: iseeyuan
Differential Revision: D31293008
fbshipit-source-id: 1c857cbd63c57b8f79c1a068789fc8605605b642
