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
Reviewed by: np
Differential Revision: https://reviews.freebsd.org/D29432

(cherry picked from commit 0a7b99553f5c1e68868e3cbaccb3160eb1a7d1d6)