Remove deprecated TxShas func from btcutil.Block.

This commit removes the previously deprecated TxShas function from
btcutil.Block.  The preferred method to access transaction hashes is via
the Sha function on each btcutil.Tx contained within the block.

For example, the following illustrates how convert the old TxShas approach
to the new method:

OLD:

for i, sha := range block.TxShas() {
	// use sha
}

NEW:

for i, tx := range block.Transactions() {
	// use tx.Sha()
}

This commit also updates the tests for the removed function.