See https://bugzilla.mozilla.org/show_bug.cgi?id=932410#c2 for the
context for which this pseudo-element was added.
In the previous patch, I had to special-case range appearance because of
this pseudo-class, but that patch makes this pseudo-class completely
redundant, as now all form controls, themed and unthemed, display
outlines, unless the native theme displays a focus indicator on its own.
Remove the special case, and make ranges use outlines like everything
else rather than this bespoke pseudo-element.
Differential Revision: https://phabricator.services.mozilla.com/D74734
