Delete defunct test/ffi directory. (#19168)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19168
ghimport-source-id: 5190a8d00c529735e99e8745c5e7cf1901fdb800
Differential Revision: D14938318
Pulled By: ezyang
fbshipit-source-id: eaeb6814178c434f737b99ae1fce63fd9ecdb432
