If you wish to remove the VisualStyler design-time skinning features then you need to disable the VisualStyler add-in. On the main Visual Studio toolbar select Tools->Add-in Manager to show the Add-in Manager dialog.

In the add-in manager dialog under available add-ins you will see an entry for the VisualStyler component, please uncheck this item in the 'Available Add-ins' column and also uncheck the 'Startup' column. Click OK, and the add-in will immediately unload.
The add-in can be reactivated at any time by checking both columns.