Remove reliable websockets configuration (#19040)

The feature is stable enough to be removed
as a config knob.

For backwards compatiblity with older mobile
clients, we keep sending the config param
as true for client config requests.

https://community-daily.mattermost.com/boards/workspace/zyoahc9uapdn3xdptac6jb69ic/285b80a3-257d-41f6-8cf4-ed80ca9d92e5/495cdb4d-c13a-4992-8eb9-80cfee2819a4/7m95frqbk9o8zxin6xo9c7iusoh

```release-note
NONE
```

app/web_hub.go

@@ -465,11 +465,7 @@ func (h *Hub) Start() {
 			case webConn := <-h.unregister:
 				// If already removed (via queue full), then removing again becomes a noop.
 				// But if not removed, mark inactive.
-				if *h.srv.Config().ServiceSettings.EnableReliableWebSockets {
-					webConn.active = false
-				} else {
-					connIndex.Remove(webConn)
-				}
+				webConn.active = false
 
 				atomic.StoreInt64(&h.connectionCount, int64(connIndex.AllActive()))