remove APG recomendation per #1156

In the interest of keeping the suggestion list uncluttered in favor of
the "best in class" products, remove the suggestion for use of APG as an
OpenPGP client in favor of just OpenKeychain. Discussion can be found on
the Github issue, but a summary of the case for this is that:

* some interfaces needed to play along with other recommendations such as
  K-9 Mail have been deprecated and it is no longer capable of doing the job in
  conjunction with other PRISM-Break recommendations

* the interface is much more complex than the alternative
  recommendation, does not conform to the Android interface guidelines
  well and thus and poses a significant usability hurdle to new users

* AGP isn't receiving regular developer attention