Purpose: small cleaning of code linked to mail.mail model in order to
avoid unnecessary overrides
* create mail: default_fetchmail_server_id in context should be sufficient
as it is the base mechanism of default_get;
* write: unused in codebase. Anyway we should not write on mail and if it
is the case giving fetchmail_server_id directly should be the correct
way of doing it;
In case of a clean_context use we might loose default_fetchmail_server_id
key. It is used notably for tracking message generation. In that case
this information could be lost. However the whole concept of propagating
a fetrchmail_server-id from incoming mail server to outgoing emails (as
mail.mail is used only for sending email) is strange and does not require
so much specific code.
Sub-part of task 1853147 (pre-cleaning before implementing mail gateway
improvements)
Linked to PR #32974
Thibault Delavallée
c87b351855