cleanup: remove pppoe_xmit() declaration.

There is no need for pppoe_xmit() forward declaration in
drivers/net/pppoe.c. This patch removes this  pppoe_xmit() declaration.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>