Investigate removing assets when component is removed from template

As raised in https://github.com/ember-cli/ember-cli/issues/6719, welcome-page currently only strips the assets from production builds.
This is because introspecting the templates to check for usage of the component might or might not be overkill.

In terms of semantics, it could also be argued that this component would function differently from other components, which might confuse users. Ideally what we want is tree shaking to be implemented ;X Should we wait/help out with that?

cc @Nippius