We decided to keep it when initially making scrollbars non-native, but I think
it's time for it to go, since it's pretty much untested.

Nobody has complained about the non-native scrollbars in chrome documents, and
the GTK scrollbar implementation is not suitable for content because it doesn't
support scrollbar-width among other things.

Differential Revision: https://phabricator.services.mozilla.com/D135670