remove LegacyDefinitions as it is empty now (#51251)
Summary:
Per title
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51251
Reviewed By: mruberry
Differential Revision: D26120574
Pulled By: ngimel
fbshipit-source-id: 223b4f358932f47e0af7413752c7db7c35402260
