oomd: move SubscriptionTaken Varlink error out of generic Varlink code

oomd: move SubscriptionTaken Varlink error out of generic Varlink code

This error is a private error returned by PID 1 to oomd. It's internal,
and very specific to the use-case. Hence it should not be part of the
org.varlink.service interface (which isn't really our namespace anyway).

Hence, let's clean this up and move it over to the ManagedOOM varlink
interface of PID, where it belongs.

Since this is a private protocol of our two daemons, and the client
(i.e. oomd) doesn't explicitly test for this error anyway we can just
move it over without ill effects.