Post Featured Image: Remove redundant condition used to determine modal class

[pierlon]

Description

This PR follows up on a change that should of been done in #14415, as @jorgefilipecosta pointed out in #14415 (review):

Both classes are the same -- I think we can just use modalClass="editor-post-featured-image__media-modal".

How has this been tested?

Built Gutenberg from source and verified the class is still shown.

Screenshots

N/A

Types of changes

Non-breaking change.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• 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.

[ntsekouras]

Thanks! 👍 - You'll have to rebase to pass CI