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/Internal/ManagedDictionary.cpp

@@ -3,7 +3,7 @@
 #include "ManagedDictionary.h"
 
 #if USE_CSHARP
-Dictionary<ManagedDictionary::KeyValueType, MTypeObject*> ManagedDictionary::CachedTypes;
+Dictionary<ManagedDictionary::KeyValueType, MType*> ManagedDictionary::CachedTypes;
 #if !USE_MONO_AOT
 ManagedDictionary::MakeGenericTypeThunk ManagedDictionary::MakeGenericType;
 ManagedDictionary::CreateInstanceThunk ManagedDictionary::CreateInstance;