Fix potential assert
Fix potential assert

It's possible that duplicate entries could be inserted into the
m_prevTxIdLookupTable deque so we have to keep looking for and deleting
entries if they still exist, when DoubleSpendProofStorage::remove() is
called.

If the duplicate entries exist and are not removed then when
we call findorphans() we may have a proof in the dequue for which
no proof actually exists in m_proofs, which then gives us an assert.

BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited