check-update-alternatives: store plain marker for using-hint

The hint used is already stored as hint-hash

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
check-update-alternatives: stop using the 'automatic' key

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>