Remove exnref and br_on_exn

[aheejin]

This removes exnref type and br_on_exn instruction.

[kripken]

a nice way to resolve a TODO 😆

[kripken]

Would this not be useful in the new exceptions code (maybe with changes)? (whatever is easier for you sgtm)

[aheejin]

It's moved here in #3498:

binaryen/src/wasm-interpreter.h

Lines 43 to 47 in 7a83bc7

	struct WasmException {
	Name event;
	Literals values;
	};
	std::ostream& operator<<(std::ostream& o, const WasmException& exn);

Previously WasmException was just a wrapper that contains a Literal of type exnref which contains a unique_ptr to ExceptionPackage which contains Name event and Literals values. Now that we don't have a exnref type Literal, I removed all those layers and WasmException directly contains the contents, which will be used only within ExpressionRunner hierarchy.

So far I haven't needed == and != operators there. I can add them if they become necessary.