Remove old autogen_magentic_one package. (#5305)

This PR removes the older `autogen_magentic_one` package, and directs
people to use the new AgentChat implementation.

Hopefully this eases confusion.

---------

Co-authored-by: Jack Gerrits <jack@jackgerrits.com>
Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>