Remove HTML Imports page

[ddbeck]

HTML Imports aren't supported and won't be in the future. This PR removes the doc for them and the one page where its linked to. The deleted page redirects to the top-level web components page instead.

Issue number that this PR fixes (if any). For example: 'Fixes #987654321'

Closes #9235

What was wrong/why is this fix needed? (quick summary only)

This is no longer supported in any browser.

Anything else that could help us review it

[github-actions]

Preview URLs

• https://pr9292.content.dev.mdn.mozit.cloud/en-US/docs/Web/HTML/Link_types

Flaws

URL: /en-US/docs/Web/HTML/Link_types
Title: Link types
on GitHub
Flaw count: 2

• broken_links:
  • Can't resolve /en-US/docs/Archive/RSS/Getting_Started/Syndicating
  • Can't resolve /en-US/docs/Permalink

External URLs

URL: /en-US/docs/Web/HTML/Link_types
Title: Link types
on GitHub

• https://bugzilla.mozilla.org/show_bug.cgi?id=1503681 (1 time)
• https://code.google.com/archive/p/shortlink/wikis/Specification.wiki (1 time)
• https://developer.apple.com/library/content/documentation/AppleApplications/Reference/SafariWebContent/ConfiguringWebApplications/ConfiguringWebApplications.html (2 times)
• https://en.wikipedia.org/wiki/Canonical_link_element (1 time)
• https://en.wikipedia.org/wiki/URL_shortening (1 time)
• Windows opened via <a target=_blank> should not have an opener by default whatwg/html#4078 (1 time)
• https://mathiasbynens.github.io/rel-noopener/ (1 time)
• https://trac.webkit.org/changeset/237144/webkit/ (1 time)
• https://www.hixie.ch/specs/pingback/pingback (1 time)

[wbamberg]

👍 thanks Daniel!