_remove_unreferenced_adapters() should only include adapters used by configuration elements

[EdGue42]

Describe the bug

When using the _cpc method export_dpm_configuration(), the resulting configuration includes storage and network adapters that are actually not referenced by other elements of the exported configuration.

Those adapters are only listed because they are referenced by storage-port resp. network-port objects.

This is not consistent with the behavior documented (and intended) for the export functionality.

Expected behavior

Unless the corresponding parameter include_unused_adapters is used, only adapters that are used by partitions / storage groups / partition links should be part of the configuration data returned by the export_dpm_configuration().