sxgbe: remove unused code

Remove the unused code of sxgbe_xpcs.

Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Suggested-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jean Sacren <sakiwit@gmail.com>
Cc: Byungho An <bh74.an@samsung.com>
Cc: Girish K S <ks.giri@samsung.com>
Link: http://lkml.kernel.org/r/alpine.DEB.2.10.1601191918470.2531@hadrien
Signed-off-by: David S. Miller <davem@davemloft.net>