keyring: Remove unused global

https://bugzilla.gnome.org/show_bug.cgi?id=708910
keyring: Cancel active prompts on disable()

Since commit 1242a16265d5bf2, we will use a fake prompt which
cancels alls requests without dialog when the keyring component
is disabled. However this does only apply to new requests, dialogs
that are already active when the session mode changes are kept
open. This is not quite as expected, so cancel the prompt in that
case.

https://bugzilla.gnome.org/show_bug.cgi?id=708910