remove unused css file : dashboard.css

In the dashboard.css file, only the following CSS rules are used :

main {
    padding: 20px;
}
@media (min-width: 768px) {
    main {
        padding-right: 40px;
        padding-left: 40px;
    }
}

To reduce the number of HTTP requests, we can remove dashboard.css file and backport the previous CSS rules to the comptoir.css file.