[FIX] mail: Clean residual context keys on _message_track_post_template

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

closes odoo/odoo#43405

Taskid: 2176445
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>