bpf: Remove xdp_do_flush_map().

xdp_do_flush_map() can be removed because there is no more user in tree.

Remove xdp_do_flush_map().

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>
Link: https://lore.kernel.org/r/20230908143215.869913-3-bigeasy@linutronix.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>