[cling] LookupHelper must not unload Transactions:

Outer RAIIs might still reference the Transaction, and unload is
assuming that it owns the transaction and can delete it / put it
into the TransactionPool.

This fixes https://github.com/root-project/root/issues/7657

We still need to track ownership as what has happened here (unload
of a Transaction held by an RAII) can happen again / elsewhere.
This will be addressed by a subsequent PR in master.