Remove adding of rel="noopener" to links with target="_blank"

[azaozz]

#43187 introduced adding of rel="noopener noreferrer" to links with target="_blank" as a security precaution. Later, in #49558 noreferrer was removed as no longer needed.

Since then most browsers were updated to imply noopener on links with target="_blank" making #43187 and similar changes not relevant any more. See:
​https://html.spec.whatwg.org/multipage/links.html#following-hyperlinks (specs),
​https://developer.mozilla.org/en-US/docs/Web/HTML/Element/a#browser_compatibility.

Adding this ticket now as a reminder to remove all noopener when all newer browsers implement the specs.

[sabernhardt]

In case anyone wonders about the comment author links on the Comments admin screen (r52007), those would keep the rel attribute because they do not open in a new tab automatically.