Remove unused `constexpr strlen` check
Remove unused `constexpr strlen` check

Summary:
Superseded by `<folly/portability/Constexpr.h>`'s `constexpr_strlen()`.
Closes https://github.com/facebook/folly/pull/585

Reviewed By: yfeldblum

Differential Revision: D4973136

Pulled By: Orvid

fbshipit-source-id: ada9bedf53ce219b0924c3f7aaad75bce9605f0f

Orvid/folly,facebook/folly,rklabs/folly,rklabs/folly,Orvid/folly,facebook/folly,facebook/folly,Orvid/folly,rklabs/folly,Orvid/folly,rklabs/folly,facebook/folly,rklabs/folly,Orvid/folly,facebook/folly