[NET]: Delete unused header file linux/if_wanpipe_common.h

Delete the unreferenced header file include/linux/if_wanpipe_common.h,
as well as the reference to it in the Doc file.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>