Remove unnecessary dependencies from FormattableContentKit

[mokagio]

In #24265 I moved more types from the apps targets into FormattableContentKit. To get this done quickly and go back to #24242, I brought in all the dependencies it needed. Those are:

• Gridicons
• WordPressKit
• WordPressUI

I believe it should be possible to replace Gridicons with custom icon(s) and SF Fonts. As for the other libraries, maybe we can extract the few objects used in Kit and UI into Shared?

See also the UIFont extension that was part of the apps targets.

[dangermattic]

Thanks for reporting! 👍