remove_trait does not remove listeners for extended traits

Consider this:
```python
from traits.api import HasTraits, Instance, Str

class Foo(HasTraits):
name = Str()

def handler(object, name, old, new):
print("Changed!", (object, name, old, new))

foo = Foo()
foo.add_trait("child", Instance(Foo))
foo.on_trait_change(handler, "child:name")
```

The following behaviour is good:
```
child = Foo()
foo.child = child
foo.child.name = "Martin"
# Changed! (<__main__.Foo object at 0x10457bb30>, 'name', '', 'Martin')
```

This is not expected:
```
foo.remove_trait("child")
child.name = "Paul"
# Changed! (<__main__.Foo object at 0x10457bb30>, 'name', 'Martin', 'Paul')
```

The listener is still gone, so adding the trait again won't bring it back, which is ~good and expected~ Edited: also unexpected.
```
foo.add_trait("child", Instance(Foo))
foo.child = Foo()
foo.child.name = "Joanna"
# Nothing is printed.
```

The second notification (where name is changed from "Martin" to "Paul") is unexpected.
While `add_trait` emits a change event on `trait_added`, `remove_trait` does not emit any events. I believe if `remove_trait` also emits an event for when a trait is removed, it would be possible for the listeners to be unhooked from the removed object.