Transaction.js: removed confusing this.class assignment in file scope

For some reason, there was a `this.class = Transaction` outside a function/object. Is this useful somehow? It looks like dead code so I removed it but I might be wrong. If it is, maybe add a comment to say what it does because it's not very idiomatic.

Edit: oops, I noticed that Script has the same assignment, so I guess it might be important....

Edit: apparently, `this` is bound to `module.exports`. But module.exports is overwritten at the last line and `ack "\.class"` didn't return anything interesting so I guess this is just dead code. Also removed it from Script.