Install VSCodium only, and remove VS Code

New machines get VSCodium alone. A migration uninstalls the VS Code
Flatpak once on existing machines; its settings stay in ~/.var/app. The
README says how to install VS Code for extensions only Microsoft's
marketplace has, such as Remote-SSH; update then leaves it alone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

install-flatpaks.sh

@@ -29,7 +29,6 @@ FLATPAKS=(
   "org.jellyfin.JellyfinDesktop"
   "app.zen_browser.zen"
   "org.localsend.localsend_app"
-  "com.visualstudio.code"
   "io.beekeeperstudio.Studio"
   "org.gnome.Boxes"
   "com.vscodium.codium"