[NET]: Kill stray reference to sock->passcred.

That struct member was deleted, but a comment
was not updated to reflect this.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/net.h

@@ -101,7 +101,6 @@ enum sock_type {
  *  @sk: internal networking protocol agnostic socket representation
  *  @wait: wait queue for several uses
  *  @type: socket type (%SOCK_STREAM, etc)
- *  @passcred: credentials (used only in Unix Sockets (aka PF_LOCAL))
  */
 struct socket {
 	socket_state		state;