[fix] drop mullvad-leta engine (#5428)

On 2025 November 27th, Mullvad will be shutting down the Leta servers.
For this reason, we also need to remove this engine from SearXNG.

[1] https://mullvad.net/en/blog/shutting-down-our-search-proxy-leta

searx/settings.yml

@@ -1426,22 +1426,6 @@ engines:
       require_api_key: false
       results: JSON
 
-  # https://docs.searxng.org/dev/engines/online/mullvad_leta.html
-  - name: mullvadleta
-    engine: mullvad_leta
-    disabled: true
-    leta_engine: google
-    categories: [general, web]
-    shortcut: ml
-
-  - name: mullvadleta brave
-    engine: mullvad_leta
-    network: mullvadleta
-    disabled: true
-    leta_engine: brave
-    categories: [general, web]
-    shortcut: mlb
-
   - name: odysee
     engine: odysee
     shortcut: od