Description of the issue/feature this PR addresses:
mailing.mailing model is reused between Mail Marketing and SMS Marketing apps, however warning message defined in Mail Marketing about incorrect mail server is not relevant for SMS Marketing.
Affected versions : 16.0+ (probably since v13.0 but older versions not maintained anymore)

Current behavior before PR:
In some cases, you can end up with a warning on SMS mailing form stating "This email from can not be used with this mail server.[...]"

For instance you can end up in this case if : 
- You have defined an ir.config_parameter mass_mailing.mail_server_id
- from_filter from this mail_server does not match email_from from user creating SMS Marketing

Desired behavior after PR is merged:
The above warning is only relevant for Mail marketing but not for SMS marketing.

This PR therefore proposes to generate warning message only if mailing_type == "mail"




---
I confirm I have signed the CLA and read the PR guidelines at www.odoo.com/submit-pr

Forward-Port-Of: odoo/odoo#205912