varlink: remove duplicated "varlink:" prefix

We had:

systemd[1]: varlink-36: New incoming message: {"method":"io.systemd.UserDatabase.GetMemberships","parameters":{"userName":"gdm","service":"io.systemd.DynamicUser"},"more":true}
systemd[1]: varlink-36: varlink: changing state idle-server → processing-method-more
systemd[1]: varlink-36: Sending message: {"error":"io.systemd.UserDatabase.NoRecordFound","parameters":{}}
systemd[1]: varlink-36: varlink: changing state processing-method-more → processed-method
systemd[1]: varlink-36: varlink: changing state processed-method → idle-server
systemd[1]: varlink-36: Got POLLHUP from socket.
systemd[1]: varlink-36: varlink: changing state idle-server → pending-disconnect
systemd[1]: varlink-36: varlink: changing state pending-disconnect → processing-disconnect
systemd[1]: varlink-36: varlink: changing state processing-disconnect → disconnected

So let's drop the "varlink:" prefix and use capitalized sentences like in other messages.

(cherry picked from commit 953394e3)