Drop dependency on PROOF/TDSet - remove GetDataset()

Remove our implementation ::GetDataset() which used to return TDSet.
Let the TGridCollection::GetDataset() (super class) throw exception
because this method shouldn't be used anyway.
1 job for fix-proof-mockup in 7 minutes (queued for 2 seconds)
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  Start
passed build-start #10365166

00:07:00