Remove tabs implementation

[mansona]

As ember-learn/ember-website#365 has gone in a different direction and we don't need a "tabs" component any more we should remove it 👍

Closes #157

[balinterdi]

Lgtm, not sure why Netlify checks are failing, though.

[mansona]

@balinterdi the Netlify build fails because they don't use npm ci for installs and you always need to "clear cache and redeploy" when working with this PR to a PR'd branch 😞 I've told them about it and they said it's a "won't fix" from their end

[balinterdi]

I see, strange. I also had the same problem with my chess project, now I know what that is.

Thanks, I'll merge the PR now.