docs: refining code of tutorial 7 routing

[thewillyhuman]

Removed the dead code from hero-detail.component.ts

Fixes #21908

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have 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
[x] angular.io application / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: #21908

What is the new behavior?

import { Component, OnInit} from '@angular/core';

export class HeroDetailComponent implements OnInit {
  hero: Hero;

}

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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