Remove unsupported annotation from header_register_callback

Summary: The callback annotation is not supported under HNI, so check manually that the
argument is a valid callback.

Fixes #4241

Reviewed By: @JoelMarcey

Differential Revision: D1687404

Signature: t1:1687404:1416345213:cd341a95ffe13f24071efb7305788b7e34b4887a