fixup! TB 2176: Rebrand Firefox to TorBrowser

TB 44998: Tidy up branding assets.

+ We drop the assets that we expect to be unused.
+ We update `about-logo.png` and `about-logo@2x.png` based on
  `about-logo.svg` using `rsvg-convert`.
+ We drop our override of the about dialog logo.
+ We add `about-logo-small.svg`, which is a copy of `about-logo.svg`
  excluding the "alpha" and "moon" bitmaps. And we use this in place of
  `about-logo.png` in `about:preferences`, `about:addons`, and other
  consumers of `moz-page-nav`.