cxgbe: remove unused linux headers

Remove unused #includes of LinuxKPI headers noticed while trying to
solve LinuxKPI struct net_device and related functions.
Neither netdevice.h nor inetdevice.h nor notifier.h seem to be needed.
This takes cxgbe(4) out of the picture of D29366.

Sponsored-by: The FreeBSD Foundation
MFC-after: 2 weeks
Reviewed-by: np
X-D-R: D29366 (extracted as further cleanup)
Differential Revision: https://reviews.freebsd.org/D29432