Remove `TestDouble.extend_onto`.

This was added in #117 to support using a module
as a pure test double by extending `TestDouble`
onto the module. Since then, I've realized we can
get the same behavior by subclassing module:

```Ruby
class MyModuleTestDouble < Module
  include TestDouble
end
```

...and that's what we do for class verifying doubles now.

Given that, there's no need to keep this API around anymore.