refactor: Make usings compatible with .NET 5+

Author: twpol

Source/Menu/MainForm.cs

@@ -31,8 +31,8 @@
 using System.Linq;
 using System.Resources;
 using System.Runtime.InteropServices;
-using System.Threading;
 using System.Windows.Forms;
+using Activity = ORTS.Menu.Activity;
 using Path = ORTS.Menu.Path;
 
 namespace ORTS

Source/Menu/ResumeForm.cs

@@ -50,7 +50,6 @@ but displayed as a thumbnail.
 
 using System;
 using System.Collections.Generic;
-using System.Diagnostics;
 using System.Drawing;
 using System.IO;
 using System.Linq;

Source/Menu/TestingForm.cs

@@ -18,17 +18,16 @@
 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
-using System.Data;
 using System.Diagnostics;
 using System.Drawing;
 using System.IO;
 using System.Linq;
-using System.Text;
 using System.Windows.Forms;
 using GNU.Gettext;
 using GNU.Gettext.WinForms;
 using ORTS.Menu;
 using ORTS.Settings;
+using Activity = ORTS.Menu.Activity;
 using Path = System.IO.Path;
 
 namespace ORTS