Remove extra RemoteClusterService instances in unit test (elastic#137647)

This refactoring is a split from elastic#137406.  A TransportService
instance already creates a RemoteClusterService instance,
with a getter method to get a reference to it.  The unit
tests were creating another instance, passing the existing
TransportService instance into the constructor.  This will
cause one time metric registration to fail since the metrics
have already been registered in the TransportService's
internal RemoteClusterService instance.

Relates: ES-12695

Author: JeremyDahlgren