Previously, the MergeNodeServiceWithCentralConfig method accepted a
ServiceSpecificRequest argument, of which only the Datacenter and
QueryOptions fields were used.
Digging a little deeper, it turns out these fields were only passed
down to the ComputeResolvedServiceConfig method (through the
ServiceConfigRequest struct) which didn't actually use them.
As such, not all call-sites passed a valid ServiceSpecificRequest
so it's safer to remove the argument altogether to prevent future
changes from depending on it.
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