Don't expect trait_modified to be in the `class_traits` dictionary

The `trait_modified` Event trait that's added to every single `HasTraits` class is useless, and I'd eventually like to remove it from Traits.

Unfortunately, that removal would currently break Mayavi: there are a couple of places in the codebase where `trait_modified` is expected to be present in the dictionary of class traits. One such is here: https://github.com/enthought/mayavi/blob/303537b8c90f2f6ae26c35d6c4a198c86fe765e4/mayavi/tools/auto_doc.py#L36-L38

It would be good to replace these pieces of code with something future-proof against the eventual removal of `trait_modified`, for example using `traits.pop("trait_modified", None)` instead of `traits.pop("trait_modified")`.

xref: enthought/traits#1097