Remove deprecated mono runtime backend

Old `mono` backend was not maintained for since 1.7 (July 2023)
`dotnet` backend still can run `mono` on AOT platforms but with the latest .NET features.

Source/Engine/Scripting/Scripting.Build.cs

@@ -15,7 +15,6 @@ public class Scripting : EngineModule
 
         if (EngineConfiguration.WithCSharp(options))
         {
-            if (EngineConfiguration.WithDotNet(options))
             {
                 void AddFrameworkDefines(string template, int major, int latestMinor)
                 {
@@ -45,12 +44,6 @@ public class Scripting : EngineModule
                     options.CompileEnv.PreprocessorDefinitions.Add("MCORE_MAIN_MODULE_NAME=" + fileName);
                 }
             }
-            else
-            {
-                // Mono
-                options.PrivateDependencies.Add("mono");
-                options.ScriptingAPI.Defines.Add("USE_MONO");
-            }
         }
 
         options.PrivateDependencies.Add("Utilities");