transaction: remove type field, set Version to 0

Two changes being done here, because they require a lot of updates to
tests. Now we're back into version 0 and we only have one type of
transaction.

It also removes GetType and GetScript interops, both are obsolete in Neo 3.

pkg/consensus/block_test.go

@@ -45,7 +45,7 @@ func TestNeoBlock_Setters(t *testing.T) {
 	b.Block.PrevHash = util.Uint256{9, 8, 7}
 	require.Equal(t, util.Uint256{9, 8, 7}, b.PrevHash())
 
-	txx := []block.Transaction{transaction.NewInvocationTX([]byte{byte(opcode.PUSH1)}, 1)}
+	txx := []block.Transaction{transaction.New([]byte{byte(opcode.PUSH1)}, 1)}
 	b.SetTransactions(txx)
 	require.Equal(t, txx, b.Transactions())
 }