Remove empty slice check (numba fixed the issue)

[NicolasHug]

numba/numba#3554 was fixed so we can remove our temporary fix from #51 once the next version is released.

Places to fix (ATM):

~/dev/pygbm » ag "array\[:0\] will"     
pygbm/splitting.py
250:        # array[:0] will cause a bug in numba 0.41 so we need the if. Remove

pygbm/utils.py
80:        # array[:0] will cause a bug in numba 0.41 so we need the if.