Move some demo-only rules to demos.css

Two things to note here. First, the rules are moved to the old demos.css
stylesheet intentionally, as these rules will probably become irrelevant
once we redesign the Demo Studio. Second, I remove the rules from
mdn-screen.css in addition to mdn-screen-migration.css so that people
not testing the migration do not have the rules applied twice.
Remove ul.index rules