feat(router-store): remove deprecated getSelectors method

[apramendorfer]

Removes deprecated getSelectors method from router-store

Closes #3813

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
[ ] Documentation content changes
[x] Other... Please describe: removes deprecated method

What is the current behavior?

Closes #3815

What is the new behavior?

The deprecated method getSelectors is removed from router-store

Does this PR introduce a breaking change?

[x] Yes
[ ] No

Other information

Migration script for the renaming of the method is already in place.

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[apramendorfer]

This is my first OS contribution, just as a disclaimer 😄

[timdeschryver]

Congratulations on your first contribution, that's awesome to hear! 🏆

[brandonroberts]

Thanks @apramendorfer, nice work!