Remove no longer supported certificates from CDDL:

[lehins]

These two certificates have already been removed from Dijkstra era:

cardano-ledger/eras/dijkstra/impl/cddl/data/dijkstra.cddl

Lines 36 to 37 in eabc9e0

	[ account_registration_cert
	// account_unregistration_cert

They need to be removed from CDDL as well. I believe this would also allow us to enable huddle spec tests for certificates in:

cardano-ledger/eras/dijkstra/impl/test/Test/Cardano/Ledger/Dijkstra/Binary/CddlSpec.hs

Lines 98 to 99 in eabc9e0

	xdescribe "fix TxCert" $ do
	huddleRoundTripCborSpec @(TxCert DijkstraEra) v "certificate"