Remove obsolete extensions pages and related code

[AlanGreene]

Changes

Related to https://github.com/tektoncd/dashboard/issues/2452

Remove the obsolete route registration, list page, and other related code for custom UI extensions. These were deprecated in v0.7.0 and support was removed in v0.19.0.

/kind misc

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• [ ] Docs included if any changes are user facing
• [ ] Tests included if any functionality added or changed
• [ ] Follows the commit message standard
• [ ] Meets the Tekton contributor standards (including functionality, content, code)
• [ ] Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
• [ ] Release notes block below has been updated with any user facing changes (new features, significant UI changes, API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• [ ] Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[tekton-robot]

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