Remove deprecated args / funcs

[calebchiam]

Description

In preparation for ConvoKit 3.0, we are removing deprecated arguments and functions, such as:

• The User component, and all user methods, e.g. has_user, get_user, iter_users, get_usernames
• The name arg in Speaker
• The root arg in Utterance
• get_info and set_info -- in favor of add_meta and retrieve_meta
• Various deprecated args in analysis transformers

Motivation and Context

This cleans up the API (see #85) and simplifies development. ConvoKit 3.0 -- aka major version update -- is the right time for these constructors / methods to be cleaned up.

How has this been tested?

Tested through CI.