[NET]: Get rid of alloc_skb_from_cache

Since this was added originally for Xen, and Xen has recently (~2.6.18)
stopped using this function, we can safely get rid of it.  Good timing
too since this function has started to bit rot.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>