Copyright headers in demo files are inconsistent

The copyright headers in the `examples/demo` files are inconsistent. Some files have outdated headers and some don't have any headers.

Q : Should the files even contain a header in the first place?
- If yes, we need to add/update them now (and put a process in place to keep them up-to-date).
- If no, we need to remove them from the files which have them.

https://github.com/enthought/traitsui/blob/3e3a05a52215e33d88fab882e0f7f27638aa8eb4/traitsui/examples/demo/Advanced/Adapted_tree_editor_demo.py#L1-L2

https://github.com/enthought/traitsui/blob/3e3a05a52215e33d88fab882e0f7f27638aa8eb4/traitsui/examples/demo/Advanced/Apply_Revert_handler_demo.py#L1-L2