docs: remove unused css file from counter component

[AlmaniaM]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

The final step in the State walkthrough contains a CSS file in the @Component decorator that has no styles and did not originally appear anywhere in the walkthrough.

Closes #2989

What is the new behavior?

The CSS file was deleted and removed from the @Component decorator.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[ngrxbot]

Preview docs changes for 6831229 at https://previews.ngrx.io/pr3002-68312298/

[timdeschryver]

Thanks @AlmaniaM