Remove std::complex to std::complex casting specialization (#37574)
Summary:
This is no longer needed because cuda copy kernel now uses `c10::complex`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37574
Differential Revision: D21328501
Pulled By: ngimel
fbshipit-source-id: dd5226e8b6c54915fb6ee52240a446f0ca30a800
