Drop "Learn More" spacing

A side effect of bugzilla bug 1872788 is that the "Learn more" links in "about:preferences" that are in a paragraph no longer have the additional spacing. Instead, there is just the default whitespace spacing.

Note that "Learn more" that appears after a checkbox or radio option still has the extra space.

Afterwards, bugzilla bug 1901194 removed the tail-with-learn-more class from some elements, since it no longer does anything. We should do the same.

@donuts is this part of some design change from mozilla? I couldn't really tell if this was part of the intention from bugzilla or phabricator.

We similarly add some spacing between the main text and a link in "about:tor", should we drop this and just have the whitespace? Moved to #43075 (closed).