[REF] mail: remove dead code in composer and lint some code bits

Purpose of this commit is to remove code coming from old features,
implementations and that was not completely cleaned.

Including

  * remove support of old "reply" composition mode which is simply a comment
    with a parent message, and is deprecated;
  * remove support of old "mail.compose.message.mode" context key that was
    used in Odoo 6.1;
  * remove support of message_id default value in context instead of
    default_parent_id;
  * remove old redirection of user / partner post. This code come from
    old age when people were able to post on their "profile" by posting
    on their user;
  * remove unnecessary method for selection field;
  * remove unused imports to clean a bit code;

We also make some optional parameters in rendering methods mandatory to
ease code understanding and future modifications.

Task ID 1853147
PR #39272

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