Purpose
=======
Clean the context to get rid of residual default_* keys
that could cause issues afterward during the mail.message
generation. Example: 'default_parent_id' would refer to
the parent_id of the current record that was used during
its creation, but could refer to wrong parent message id,
leading to a traceback in case the related message_id
doesn't exist
closesodoo/odoo#43830
Taskid: 2176445
X-original-commit: 82d2d581a2590a9c782cbc71b5005a4198b70d77
Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>