Remove the `blocklyDropdownMenu` CSS class, and move associated styling

[BeksOmega]

The blocklyDropdownMenu class indicates the the dropdown div is owned by a dropdown field, so that it can be styled accordingly. We should delete this, and move the styling to be based on #8311, and #8322

[AbhinavKRN]

hey @BeksOmega , can you assign this issue to me!

[BeksOmega]

Heya @AbhinavKRN , sadly this isn't ready to be worked on yet because #8311 and #8322 aren't completed :/