[FIX] mass_mailing: hide the Mail Debug tab

The Mail Debug tab is confusing for users and should only be used by
people who know what can or cannot be expected from it. It is not
providing a good preview of the e-mail's rendering since the HTML it
displays is not meant to be rendered by a browser but by all sorts of
e-mail clients. This means that some things that render properly in it
won't necessarily render properly in some e-mail clients, but also
likewise that some things that don't render properly in it will in fact
render as expected in e-mail clients. This is why it's only available in
debug mode but since many users routinely enable debug mode to get
access to hidden features, this is not enough to prevent them from
reporting issues that are not issues.

closes odoo/odoo#132284

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/mass_mailing/static/src/scss/mass_mailing.scss

@@ -11,6 +11,10 @@
         margin-right: -$o-sheet-cancel-hpadding;
         margin-bottom: -40px;
     }
+
+    .o_notebook .o_notebook_headers .nav-link[name="mail_debug"] {
+        display: none;
+    }
 }
 
 @include media-breakpoint-up(lg, $o-extra-grid-breakpoints) {