net: sock: remove excess structure-member documentation

Remove a couple of kerneldoc entries for struct members that do not exist,
addressing these warnings:

  ./include/net/sock.h:548: warning: Excess struct member '__sk_flags_offset' description in 'sock'
  ./include/net/sock.h:548: warning: Excess struct member 'sk_padding' description in 'sock'

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