Fix marker-text-combine-upright.html.
This test was failing because Firefox sets white-space:pre on ::marker,
but the reference wasn't taking it into account. So this patch just
uses all:unset to undo whatever random things browsers may decide to
set on ::marker.
The case with a symbolic marker wasn't working either but I guess it's
fine because browsers are allowed to paint an image of their choice
so the interaction with text-combine-upright seems hard to tell.
So I'm just removing that case.
Differential Revision: https://phabricator.services.mozilla.com/D223646
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1921113
gecko-commit: 12bf64fef14a73d5df0de2ba5876f3eeb28ec339
gecko-reviewers: emilio