remove DeprecatedOptionalField
remove DeprecatedOptionalField

Summary: `DeprecatedOptionalField` is obsolete, it's not used anywhere

Reviewed By: yfeldblum

Differential Revision: D22500445

fbshipit-source-id: 0bec819519674cfa2cc24d50d8110b0fb3b6c296

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift