Remove FixedString::hash()
Remove FixedString::hash()

Summary: This is not the canonical way we do hashes in C++. In addition, this API is unused and untested.

Reviewed By: yfeldblum

Differential Revision: D36435408

fbshipit-source-id: 509b68766beb009909187edb4a78d919f8370dbd

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly