Remove logic and existing references to the iPad app

[cjg89]

The iPad app hasn't been a thing for a while. We have some leftover settings, markup and js logic still floating around for it that are still impacting frontend markup; the old "available on iPad!" modal in particular is getting flagged in Siteimprove for not being within an ARIA landmark. Removing these should help resolve those accessibility issues and help reduce a little bit of cruft in the theme and WP admin.