remove ZStrong as a fbcode_builder user

Summary:
X-link: https://github.com/facebookincubator/zstrong/pull/1266

ZStrong is moving away from fbcode_builder to use vanilla CMake. Removing this manifest so getdeps can stop considering the repo.

Reviewed By: terrelln

Differential Revision: D72397364

fbshipit-source-id: 5cbbd6edebe46a29aeddd93d848ad5a45e8d0aad

Author: Victor-C-Zhang