Move existing instance removal from init to render

If the instance isn't set to append, other instances on the target element should first be removed/torn down.

I don't foresee any issues with moving this, and all the tests are happy. I'd like to get another set of eyes and cerebral cortices on it, though.

Fixes #1430
@evs-chris I've kept with @Rich-Harris' habit of inlining assignment with the `if` in these situations:
```js
if ( el = getElement( ractive.el ) ) {
```