remove c#7 features for compatibility with 2015 Build Tools

https://github.com/godotengine/godot/issues/15742

modules/mono/glue/cs_files/DebuggingUtils.cs

@@ -34,8 +34,8 @@ namespace Godot
 
             StringBuilder sb = new StringBuilder();
 
-            if (methodBase is MethodInfo methodInfo)
-                sb.AppendTypeName(methodInfo.ReturnType);
+            if (methodBase is MethodInfo)
+                sb.AppendTypeName(((MethodInfo)methodBase).ReturnType);
 
             sb.Append(methodBase.DeclaringType.FullName);
             sb.Append(".");