remove new reddit platform handling

[eritbh]

Aside from the "open in new reddit" button that was already handled in #1025, there's not actually a whole lot of dead code we can remove by not supporting new Reddit. We still rely on the combination of TBListener + the oldreddit module for a bunch of UI stuff in old Reddit; eventually #940 will replace both with a new system and everything will be changed to use that instead, but that's not happening here. A couple features specific to new Reddit were removed, and the modbar overlayFromBarRedesign setting (which controls whether clicking queue buttons in the modbar on new reddit opens the queue as an overlay or acts as a normal link) now applies to shreddit instead.

Fixes #1027

[eritbh]

branch updated to actually remove the lightbox setting and purge it from storage. Also cleaned up the related purges of removed settings - located them all together in a single Promise.all, and makes sure that they're purged regardless of the version we're updating from, since it's cheap to delete things even if they already don't exist