Revert changes in AppBuilder.cs

src/Avalonia.DotNetCoreRuntime/AppBuilder.cs

@@ -47,6 +47,8 @@ namespace Avalonia
             //that CLR doesn't try to load dependencies before referencing method is jitted
             if (os == OperatingSystemType.WinNT)
                 LoadWin32();
+            else if(os==OperatingSystemType.OSX)
+                LoadMonoMac();
             else
                 LoadGtk3();
             this.UseSkia();
@@ -54,7 +56,7 @@ namespace Avalonia
             return this;
         }
 
-        
+        void LoadMonoMac() => this.UseMonoMac();
         void LoadWin32() => this.UseWin32();
         void LoadGtk3() => this.UseGtk3();
     }