[FIX] mass_mailing: respect minimal use of 'important' in design tab

Some styles added by the design tab require the use of the 'important'
keyword. Prior to this that keyword was added to all styles of a
selector that contained at least one priority style.

task-2821155

X-original-commit: e41de41c6f1e8d3f34368e191b9debc03c57ab05
Part-of: odoo/odoo#92138

addons/mass_mailing/static/src/js/mass_mailing_snippets.js

@@ -265,7 +265,7 @@ options.registry.DesignTab = options.Class.extend({
                     for (const style of rule.style) {
                         const ownPriority = rule.style.getPropertyPriority(style) ? ' !important' : '';
                         if (style !== params.cssProperty) {
-                            cssTexts.push(`${style}: ${rule.style[style]}${priority || ownPriority};`);
+                            cssTexts.push(`${style}: ${rule.style[style]}${ownPriority};`);
                         }
                     }
                     cssTexts.push(`${params.cssProperty}: ${value}${priority};`);