Adwaita: Fix padding/rounding on submenus

• Apply menubar submenu style for .csd submenus;
• Remove child subemenu style (looks like it is inherited from a parent).

Closes #1976 (closed)

cc @jimmac

I did some experiments, and it looks like child submenu inherits style from parent (if unset), so I removed it.

Before / after (lightblue background applied on submenus for better visibility, and ignore red square - missing assets):