Remove template altogether if nothing is retained

[Rich-Harris]

Since certain properties (namespace, components, and types if we do #418) are removed from the generated code, we could get rid of template if it doesn't have any other properties:

<Foo/>

<script>
  import Foo from './Foo.html';

  export default {
    components: { Foo }
  };
</script>

That currently leaves the rather cryptic

var template = (function () {
  return {
    };
}());

[Rich-Harris]

Just want the compiled output to be as clean as possible — a lot of people will make a judgement about whether or not to use Svelte based on the output code they see in the REPL, and having gubbins left lying around like that looks messy. (This actually makes our lives harder, since people don't normally read the entire source code of React or Vue — the nearest equivalent — before committing, but I think it's important for people to see.)

Besides, most people don't use CC — UglifyJS leaves you with var a=function(){return{}}();.

[PaulBGD]

Yep, I removed my comment for those reasons.