net: remove net_device operation ndo_xdp_flush

All drivers are cleaned up and no references to ndo_xdp_flush
are left in drivers, it is time to remove the net_device_ops
operation ndo_xdp_flush.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>