Remove unused blocks-font-size classname

[talldan]

Description

This is a very tiny PR that removes a classname blocks-font-sizes from the paragraph block's Text Settings panel.

The css for this classname seem to have been removed a while ago from the paragraph block, so removing it has no visual effect. The only usage was in an end-to-end test where it was used as part of a selector, but this was easily replaced with the font-size-picker classname.

How has this been tested?

1. Add a paragraph block
2. Try changing the font size
3. Observe that the font size picker looks the same as it did previously

Types of changes

Non-breaking task

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. .

[draganescu]

I have tested this. Initially I found the "default" setting to be missing but after a rebase everything was OK. All good codewise.