wangle/cmake: Remove directory that does not exist
wangle/cmake: Remove directory that does not exist

Summary:
portability dir doesn't exist. Remove, otherwise it causes problems for
other things, e.g. CPack

Reviewed By: yfeldblum

Differential Revision: D22987671

fbshipit-source-id: 9120088e4e0db9d7d1ee7ea19d02965e458c9d47

facebook/wangle,facebook/wangle,facebook/wangle