Remove forward compat code for serialization format (#25440)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25440
See the comments deleted for what this PR is all about
Test Plan: Imported from OSS
Differential Revision: D17125690
Pulled By: suo
fbshipit-source-id: a4a2f541a3e161f9c15b51df475130e7bf683cf8
