Update checkBIP0030 to not use deprecated function.

Rather than using the deprecated TxShas function on a btcutil.Block,
convert the checkBIP0030 to use the newer preferred method of ranging over
the Transactions to obtain the cached hash of each transaction.

This is also a little more efficient since it can avoid creating and
caching an extra slice to keep the hashes in addition to having the hash
cached with each transaction.