Remove `WPStyleGuide+Notifications.swift` from FormattableContentKit

[mokagio]

See @kean's comments from #24265

• Move some content group and styles to FormattableContentKit #24265 (comment)
• Move some content group and styles to FormattableContentKit #24265 (comment)

Once #24165 is complete, we have to follow up and clean up the boundaries.

[dangermattic]

Thanks for reporting! 👍