Bug 1833403 - Simplify some appearance code. r=stransky,mac-reviewers,handyman,devtools-reviewers,mstange

I was looking into simplifying our scrollbar styles:

 * StyleAppearance::Resizer is not used in content, and some of the
   values were only for <xul:window> which are not supported anymore.

 * Statusbarpanel and Resizerpanel aren't used. Statusbar is only used
   once on macOS so we only need to keep it there.

Differential Revision: https://phabricator.services.mozilla.com/D178374