Clean up trait definitions on toolkit implementation classes

Because the various mixin classes were not necessarily `HasTraits` classes until recently, a lot of toolkit implementation classes duplicate the trait definitions from the mixin. This shouldn't be needed unless there is something toolkit-specific needed for the trait in the concrete class (eg. it is storing a toolkit object) or (very) occasionally if there are issues because of the way that Traits resolves class traits from super classes when there is multiple inheritance.

We should remove all duplicated trait definitions that we can.