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
```

api4/websocket_test.go

@@ -204,10 +204,6 @@ func TestWebSocketReconnectRace(t *testing.T) {
 	th := Setup(t).InitBasic()
 	defer th.TearDown()
 
-	th.App.UpdateConfig(func(cfg *model.Config) {
-		*cfg.ServiceSettings.EnableReliableWebSockets = true
-	})
-
 	WebSocketClient, err := th.CreateWebSocketClient()
 	require.NoError(t, err)
 	defer WebSocketClient.Close()