net: skbuff: Remove some excess struct-member documentation

Remove documentation for nonexistent structure members, addressing these
warnings:

  ./include/linux/skbuff.h:1063: warning: Excess struct member 'sp' description in 'sk_buff'
  ./include/linux/skbuff.h:1063: warning: Excess struct member 'nf_bridge' description in 'sk_buff'

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>