docs: remove old elements item from navigation

[jenniferfell]

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)
• [ X] 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)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] angular.io application / infrastructure changes
[X ] Other... Please describe: Docs navigation change

What is the current behavior?

There is an extra old "Custom Elements" item in the navigation, with no associated file. This change removes that old entry. (new nav entry was in the elements doc PR)

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[jenniferfell]

PR #22966 added a new custom element item to the nav, with the new doc associated. It removed a page of old doc, but it failed to remove the old nav item. This PR removes that old nav item.

[jenniferfell]

Fixed by #23175

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