We generate a Hash method for any hashable mojom struct, where a mojom struct is hashable iff all its fields are hashable. We only had this because we used to use std::unordered_map to represent mojom maps in C++ bindings.

Now we use base::flat_map which only requires operator<. And while we probably could generated operator< for generated structs and arrays, we clearly haven't had a need for it yet (typemapped types like String16->base::string16 will just work as keys already because only the typemapped type needs an operator<).

So let's delete all the Hash code, including mojo/public/cpp/bindings/hash_util.h and all of the generated code which uses it.