net: skb: remove unused asserts

We are discouraging the use of BUG() these days, remove the
unused ASSERT macros from skbuff.h.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>