Let device pick its own frame rate ranges

I removed a segment of code that seemed to be arbitrarily picking a framerate range.

This was causing a crash that a previous version of PBJVision wasn't having an issue with. Not sure what introduced it, or whether it's a coincidence. 

[The docs warn of this particular crash](https://developer.apple.com/library/ios/documentation/AVFoundation/Reference/AVCaptureDevice_Class/Reference/Reference.html#//apple_ref/occ/instp/AVCaptureDevice/activeVideoMinFrameDuration), and it doesn't seem like this bit of code is necessary for any other functionality, as far as I can tell.