Remove support for MSC3575 (sliding sync)

Co-authored-by: Olivia Lee <olivia@computer.surgery>

src/api/well_known.rs

@@ -37,14 +37,5 @@ pub(crate) async fn client(_: Ar<client::Request>) -> Ra<client::Response> {
     Ra(client::Response {
         homeserver: client::HomeserverInfo::new(base_url.clone()),
         identity_server: None,
-        sliding_sync_proxy: services()
-            .globals
-            .config
-            .server_discovery
-            .client
-            .advertise_sliding_sync
-            .then_some(client::SlidingSyncProxyInfo {
-                url: base_url,
-            }),
     })
 }