xdp: done implementing ndo_xdp_xmit flush flag for all drivers

Removing XDP_XMIT_FLAGS_NONE as all driver now implement
a flush operation in their ndo_xdp_xmit call.  The compiler
will catch if any users of XDP_XMIT_FLAGS_NONE remains.
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/net/xdp.h

@@ -41,7 +41,6 @@ enum xdp_mem_type {
 };
 
 /* XDP flags for ndo_xdp_xmit */
-#define XDP_XMIT_FLAGS_NONE	0U
 #define XDP_XMIT_FLUSH		(1U << 0)	/* doorbell signal consumer */
 #define XDP_XMIT_FLAGS_MASK	XDP_XMIT_FLUSH