Drop static SGVector::resize().

This method can be misleading since it should not be used for SGVectors,
use SGVector::resize_vector() instead.

For example I think that this should be reasonable:
SGVector<int32_t> vi(5);
SGVector<int32_t>::resize(vi.vector,vi.vlen, 10);
However, it is a bug since resize() doesn't modify the member vlen.
Then, I think we are better without this method.