Remove unreachable code

[steveklabnik]

This panic can never occur, so putting it here isn't useful.

[jennamagius]

Yeah, I put that there on purpose, because it's kinda non-local that std::process::exit() has return type !. But then since then I've used std::process::exit() in many, many more places without this extra explicitness, so may-as-well just normalize.

My fuzzy memory suggests that there's one or two more explicit panics after a -> ! function somewhere in this code base.

[steveklabnik]

Cool cool, makes sense :)