MM-34125: Remove WebSocketDelay feature flag (#17441)

It has been soaked for enough time in community-daily
and also in community. Enabling it for all.

```release-note
TCP_NO_DELAY is disabled for websocket connections
to allow for higher throughput
```

https://mattermost.atlassian.net/browse/MM-34125

app/web_conn.go

@@ -71,8 +71,9 @@ func (a *App) NewWebConn(ws *websocket.Conn, session model.Session, t i18n.Trans
 		})
 	}
 
-	if a.srv.Config().FeatureFlags.WebSocketDelay {
 	// Disable TCP_NO_DELAY for higher throughput
+	// Unfortunately, it doesn't work for tls.Conn,
+	// and currently, the API doesn't expose the underlying TCP conn.
 	tcpConn, ok := ws.UnderlyingConn().(*net.TCPConn)
 	if ok {
 		err := tcpConn.SetNoDelay(false)
@@ -80,7 +81,6 @@ func (a *App) NewWebConn(ws *websocket.Conn, session model.Session, t i18n.Trans
 			mlog.Warn("Error in setting NoDelay socket opts", mlog.Err(err))
 		}
 	}
-	}
 
 	wc := &WebConn{
 		App:                a,

model/feature_flags.go

@@ -34,8 +34,6 @@ type FeatureFlags struct {
 
 	// Toggle on and off support for Files search
 	FilesSearch bool
-	// Feature flag to control setting the TCP_NO_DELAY setting for websockets.
-	WebSocketDelay bool
 
 	// Control support for custom data retention policies
 	CustomDataRetentionEnabled bool
@@ -52,7 +50,6 @@ func (f *FeatureFlags) SetDefaults() {
 
 	f.PluginIncidentManagement = "1.7.0"
 	f.PluginApps = ""
-	f.WebSocketDelay = false
 	f.CustomDataRetentionEnabled = false
 }