Revert "Add Anki Sync Server addon (self-hosted flashcard sync)"

This reverts commit 75baab8914.

Readme.md

@@ -1291,17 +1291,6 @@ terminal menu and the web UI, so they can't drift apart).
   steps, so any code or hardware-config change picked up by the pull
   actually takes effect. Also offers an on-demand Electron version
   check. See "Recent Updates (v2.15.0)" below.
-- `menus/addon_anki_sync.sh` — **Anki Sync Server** (Addons): installs
-  the official self-hosted Anki sync server (`python -m
-  anki.syncserver`, from the `anki` PyPI package, no Qt/GUI deps) in
-  its own venv under `$ANKI_SYNC_DIR`, run as a systemd service under
-  `$KIOSK_USER`, same shape as the Web UI's Node app. Lets Anki desktop/
-  AnkiDroid clients on the LAN sync flashcards (simple vocab/math-fact
-  decks or complex custom note types) against this kiosk instead of
-  AnkiWeb; supports multiple sync accounts (`SYNC_USER1`, `SYNC_USER2`,
-  ...). Quizlet sets still go through Anki's own tab-separated import
-  first — this server only syncs the resulting collection, same as it
-  would with AnkiWeb.
 - `webui/` + `menus/addon_webui.sh` — **Web UI** (Addons, installed by
   default during provisioning): a small Node/Express app running as a
   systemd service under `$KIOSK_USER` with zero ambient `sudo`, giving