Simplify `simple.smithy` (#2339)

`simple.smithy` is currently not that simple. Besides, it serves no
effective purpose: everything that the model exercises is already
exercised in a clearer and more orderly manner in other integration
tests.

This PR makes the model minimal, so that it recovers its purpose as a
simple smoke test. Contributors can also use the model as a scratchpad
for features or bugfixs they're working on.