Reject invalid subclassing of RLMObjects at schema generation time

Rather than producing weird errors and bizarre behavior when they're actually used.

Remove the unit test which was testing them since there's no way to have an invalid subclass for just a single test.

@alazier 
This whole method could be rewritten from here:
```
if (class != RLMObject.class && RLMIsSubclass(class, RLMObject.class)) {
 @throw...
}