fix: Nuke all remaining references to MSC3575 in docs and code

docs/advanced/delegation.mdx

@@ -109,9 +109,6 @@ Restart Continuwuity and your reverse proxy. Once that's done, visit these route
 {
   "m.homeserver": {
     "base_url": "https://matrix.example.com/"
-  },
-  "org.matrix.msc3575.proxy": {
-    "url": "https://matrix.example.com/"
   }
 }
 ```

docs/public/.well-known/matrix/client

@@ -1 +1 @@
-{"m.homeserver":{"base_url": "https://matrix.continuwuity.org"},"org.matrix.msc3575.proxy":{"url": "https://matrix.continuwuity.org"},"org.matrix.msc4143.rtc_foci":[{"type":"livekit","livekit_service_url":"https://livekit.ellis.link"}]}
+{"m.homeserver":{"base_url": "https://matrix.continuwuity.org"},"org.matrix.msc4143.rtc_foci":[{"type":"livekit","livekit_service_url":"https://livekit.ellis.link"}]}