Hide private browsing icon in toolbar on OS X

The VerticalTabs toolbar contains a private browsing indicator icon (also note the incorrect positioning of the icon):

![vt](https://cloud.githubusercontent.com/assets/1319028/4782296/9bef7a90-5cea-11e4-8d27-1c39f447b28d.png)

However, OS X already has a private browsing indicator icon in the window titlebar. Therefore, we should hide the icon entirely in the VerticalTabs toolbar.