Remove dead code in CTInnerNode#refine

[FredrikTaquist]

The following code

		for (ShortPrefix u : shorts) {
			if (!(u instanceof ShortPrefix)) {
				leaves.get(u).elevatePrefix(u, oracle, inputs);
			}
		}

in the method CTInnerNode#refine does not do anything since it will never be the case that u is not an instance of ShortPrefix. The intent is to make sure that prefixes which are short remain short after being sifted into the classification tree following refinement. Since this is already handled by ClassificationTree#refine, it is not necessary to have that check here. This PR therefore removes the code.

[kostis]

LGTM.