[IMP] mail: remove confusing readonly=False on partner_email

This field was never intended to be editable, and indeed all the views
on mail.channel.partner I could find either don't list the field at
all, or explicitly make it readonly.

The editability is actually an artefact from when related fields were
changed from editable by default to readonly by default, all existing
fields were bulk-set to `readonly=False` in
3f4f77fd with the intent of eventually
reviewing this status. That eventuality apparently never came to be,
and so most of those fields are probably flagged `readonly=False` for
no reason to this day.

closes odoo/odoo#126641

Signed-off-by: Louis Wicket (wil) <wil@odoo.com>