Remove structseq_slice (#35625)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35625
Python 2 has reached end-of-life and is no longer supported by PyTorch.
This function was already ifdef'ed out in Python 2.
Added a comment about when we might be able to remove this entire file.
Test Plan: CI
Differential Revision: D20842885
Pulled By: dreiss
fbshipit-source-id: 1fd3b1b2ff5a82caaf3bc11344dde2941427cfc0
