[w3c/manifest] Fix broken specs (#815)

Fixes some of the issues in [#735](https://github.com/w3c/manifest/issues/735)

This change (choose one):

* [ ] Breaks existing normative behavior (please add label "breaking")
* [ ] Adds new normative requirements
* [ ] Adds new normative recommendations or optional items
* [ ] Makes only editorial changes (only changes informative sections, or
  changes normative sections without changing behavior)
* [ ] Is a "chore" (metadata, formatting, fixing warnings, etc).

Commit message:
- update 301 links to their final redirect
- remove unused specs
- update spec references using https://respec.org/xref/

@marcoscaceres I've opened `index.html` locally on this branch. There are no errors. There are some specs that I could not find in xref and removing them resulted in errors.  Thus, I have left them in. I've PMed the list to you.

Please let me know what to do next. Thanks!
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/815

-- Commit Summary --

  * update 301 links to their final redirect
  * remove unused specs
  * update external resource link spec
  * fix active document spec
  * fix WorkerType spec
  * remove unused specs
  * fix rel attribute spec
  * fix media attribute spec
  * fix event handler idl attribute spec
  * remove unused specs

-- File Changes --

    M index.html (112)

-- Patch Links --

https://github.com/w3c/manifest/pull/815.patch
https://github.com/w3c/manifest/pull/815.diff

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