Remove potential fatal mistake

This pull request removes a potentially fatal mistake by a developer implicit in the current API where they can generate a private key, and never recover it.

There is no reason to include this feature in the API, if the developer wishes to throw away a private key, let them do it explicitly:

```
var uselessPubKey = new ECKey('1111...').getPub()
```

As it stands, this just seems potentially dangerous with no real benefit.