Seahorse/GNOME keyring take over the GnuPG agent role

Originally created by @intrigeri on #7330 (Redmine)

For OpenPGP smartcard support, we’ve moved away from using Seahorse/GNOME keyring as the GnuPG agent.
However, on 1.1~beta1, this change seems to be reverted. Likely a migrated (or unmigrated?) GConf setting is wrong.

Feature Branch: bugfix/7330-disable-gnome-keyring-gpg-functionality

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