Remove `trait_modified`

Every `HasTraits` subclass comes with a `trait_modified` Event trait. However, this trait is never fired by anything in ETS, and is never observed by any ETS-using project code available to me.

Nevertheless, removing this could break existing code that grabs the `traits` dictionary from an object and then explicitly deletes the `"trait_added"` and `"trait_modified"` names because they're not interesting. Such code definitely exists. The fix for that code should be trivial, but it'll still need fixing.

So I think we can't actually _remove_ this until 7.0.0. I can't think of a way to noisily deprecate it in the code before then, but we could advertise the planned removal in a prior release (6.1 or 6.2).