Super Cache: update mobile device support docs and support

Jetpack no longer supports mobile themes, but WPSC still has a helper plugin to allow caching to work with it.
The plugin FAQ also lists this feature and two plugins that have been closed for many years. We must update that text.

It appears that WPTouch is the only plugin offering a mobile theme.

We need to remove that helper plugin and update the docs to only include links to WPTouch.
Links:
https://wordpress.org/plugins/wp-super-cache/#how%20do%20i%20serve%20cached%20mobile%20pages%20to%20clients%20on%20small%20screens%20like%20phones%20and%20tablets%3F
https://jetpack.com/support/wp-super-cache/initial-wp-super-cache-setup-and-configuration/ doesn't mention mobile support at all. The FAQ link on this setting should possibly go to the previous link.

Start by searching the repository for the mobile-theme helper plugin, the FAQ text, and the setting's FAQ link; review the linked WordPress.org and Jetpack pages for current wording. Done means the helper is removed and the FAQ/support text mentions only WPTouch, with the setting link pointing to the appropriate mobile-support page.

Written by the indexing model from the issue text.