qobject: Drop qstring_get_try_str()

No users left outside tests/, and the ones in tests/ can just as well
use qstring_get_str().  Do that, and drop the function.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201211171152.146877-14-armbru@redhat.com>