Remove unused CUDA kernels

[una-dinosauria]

Some CUDA code is never called from julia, which is confusing, eg

Rayuela.jl/deps/src/cudautils.cu

Lines 85 to 95 in ccf22ab

	__device__ void _veccost(
	float *d_rx, // data to use (X)
	float *d_codebooks, // codebooks (C)
	unsigned char *d_codes, // the codes (B)
	float *d_veccost, // where to save the cost
	int m, // number of codebooks
	int n) { // number of vectors in X
	// FIXME hard-coding 256 entries in each codebook, and 128 dimensions
	const int H = 256; // size of each codebook
	const int D = 128; // dimensionality of each vector

as reported in #39. We should remove unused code in general.