net: skb: remove old comments about frag_size for build_skb()

Since commit ce098da1 ("skbuff: Introduce slab_build_skb()")
drivers trying to build skb around slab-backed buffers should
go via slab_build_skb() rather than passing frag_size = 0 to
the main build_skb().

Remove the copy'n'pasted comments about 0 meaning slab.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>