Delete core FunctionRef (#3452)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3452

It was previously replaced with a function pointer typedef; let's just use function pointers directly and have one less type to worry about.
ghstack-source-id: 224871157

Reviewed By: larryliu0820

Differential Revision: D56846841

fbshipit-source-id: 6ccb11ddee2b1f6a310a027b493233788d9ec706