Remove potential for private key loss

This pull request removes a potentially fatal mistake that could be made by a developer using the current API.
It removes the implicit private key generation for the `ECPubKey` with an empty constructor; with which they can generate and never a recover private key.

There is no reason (that I can see) to include this feature in the API, if the developer wishes to throw away a private key, let them do it explicitly?

```
var lonelyPubKey = new ECKey().getPub()
```

As it stands, this just seems potentially dangerous with no real benefit.