[IMP] website: optimize CSS for mobile navbar background color

The `o-bg-color` mixin was used to color the mobile navbar: it was
useless. First, in general, `o-bg-color` should rarely be used:
`o-apply-color` is best as it will call the other one when needed. Then,
this comes with many extra rules to color the text, inner subtitle, etc.
All of those rules are already inherited from the parent navbar: we only
want to re-apply the background-color on a sub element (the mobile
navbar) in this case.

Part-of: odoo#141198

addons/website/static/src/scss/website.scss

@@ -359,11 +359,8 @@ $-seen-urls: ();
 
 // Apply background color to the offcanvas menu
 .o_navbar_mobile {
-    $-menu-color: o-color('menu-custom') or o-color('menu');
-
-    @include o-bg-color($-menu-color);
-
     height: 100dvh;
+    background-color: o-color('menu-custom') or o-color('menu');
 }
 
 $-header-nav-link-height: $nav-link-height;