refactor(Example): remove ngrx-store-freeze

[timdeschryver]

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)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Closes #

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

When #1613 gets merged we have mutation checks built in

[ngrxbot]

Preview docs changes for cba6676 at https://previews.ngrx.io/pr1633-cba6676/

[ngrxbot]

Preview docs changes for cba6676 at https://previews.ngrx.io/pr1633-cba6676/

[timdeschryver]

Since runtime checks are now opt-in, we should enable the immutably check when #1613 gets merged.