Fix duplicate custom classnames in navigation submenu block

[talldan]

Description

Fixes #36464

This is a pretty straightforward fix. The get_block_wrapper_attributes already calls the custom class name hook to add the classnames to the rendered block markup. It seems the nav submenu block also had some custom code that did the same, likely this was copied from the navigation link block, as the same issue was fixed there in #35634.

That custom code isn't needed so can be removed.

How has this been tested?

1. In the Editor, add a submenu block
2. Add a class for the submenu block in Advanced > Additional CSS class(es)
3. Save your page
4. Go to the live page and inspect the submenu block
5. The classname should only be output once

Screenshots

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[Mamaduka]

Thanks, @talldan.

Changes look good and work as expected.