[REM] crm, project: remove support of mail-based record update

Currently crm and project parse incoming email body following an old
syntax to find some "set field - value" lines in the email content. It is
used to update the record linked to the discussion directly from the email.

However this feature is undocumented and not really supported anymore.
Moreover we don't think being able to update records from emails is a
reliable way of working. Moreover those update is limited to

 * revenue and probably for leads; this should totally be managed from
   the pipeline view by lead / opportunity responsible;
 * priority update coming from a "priority" entry in the incoming email;
   this code seems strange as priority is probably not standard in email
   headers;
 * planned hours for project; this should be managed by project user
   and managers through dedicated views in Odoo;

Let us remove that old feature and let people use desktop or mobile
interface to update their records.