fixup! TB3: Tor Browser's official .mozconfigs.

Bug 42398: Include the channel in MOZ_APP_DISPLAYNAME

This variable is defined in the branding configuration, no need to
repeat it in the mozconfig.

browser/config/mozconfigs/tor-browser

 . $topsrcdir/browser/config/mozconfigs/base-browser
 
-mk_add_options MOZ_APP_DISPLAYNAME="Tor Browser"
 # If building outside tor-browser-build, you might want to customize this value.
 # On Linux, it influences the WM_CLASS value, which is shown on GNOME's ALT-TAB,
 # for example.