Fixes U4-1619 so MediaTypes are deleted the same way as DocumentTypes using the legacy api/backoffice.

src/Umbraco.Core/Services/ContentTypeService.cs

@@ -358,7 +358,7 @@ namespace Umbraco.Core.Services
 	        if (DeletingMediaType.IsRaisedEventCancelled(new DeleteEventArgs<IMediaType>(mediaType), this)) 
 				return;
 	        
-			_mediaService.DeleteMediaOfType(mediaType.Id);
+			_mediaService.DeleteMediaOfType(mediaType.Id, userId);
 
 	        var uow = _uowProvider.GetUnitOfWork();
 	        using (var repository = _repositoryFactory.CreateMediaTypeRepository(uow))

src/umbraco.cms/businesslogic/media/MediaType.cs

@@ -150,10 +150,13 @@ namespace umbraco.cms.businesslogic.media
 
             if (!e.Cancel)
             {
-                // delete all documents of this type
-                Media.DeleteFromType(this);
-                // Delete contentType
-                base.delete();
+                // check that no media types uses me as a master
+                if (GetAllAsList().Any(dt => dt.MasterContentTypes.Contains(this.Id)))
+                {
+                    throw new ArgumentException("Can't delete a Media Type used as a Master Content Type. Please remove all references first!");
+                }
+
+                ApplicationContext.Current.Services.ContentTypeService.Delete(_mediaType);
 
                 FireAfterDelete(e);
             }