remove_trait()'s behavior on non-dynamically added traits is undocumented and poorly defined

`remove_trait()` is supposed to be used for traits that were added via `add_trait()`, however this is neither specified in the docstring

https://github.com/enthought/traits/blob/eb0027f3d6a57968104949e5a5fc5d7db9d4a577/traits/has_traits.py#L2842-L2854

or elsewhere in the documentation, nor enforced in the code.

As a result, `remove_trait()` can be performed with a "static" trait, in which case it erases the trait's value, keeps the trait, and returns `False`. This behavior should probably be reconsidered or documented.