What happens to frames when you speed up a video clip?
In the simplest case, half the frames -- every second frame, or all the even (or odd) numbered original frames -- are just gone, discarded. The frame rate of the resulting video does not change, it's still 60 fps.

It's possible to create a video where each new frame is a blend of two original frames but this is unusual and often not effective. Or in a field-based system where each original frame is composed of two distinct half vertical resolution images, you can discard half the fields instead of half the frames. But still, the result will be 60 fps.  

If you were working in a 60 fps environment and imported a 30 fps clip, you could effectively double the frame rate of the original clip, but that isn't what you asked about.