xdp: remove KNOT_XDP_LOAD_BPF_NEVER mode, which is not used

src/libknot/xdp/bpf-user.c

@@ -226,14 +226,6 @@ int kxsk_iface_new(const char *if_name, int if_queue, knot_xdp_load_bpf_t load_b
 
 	int ret;
 	switch (load_bpf) {
-	case KNOT_XDP_LOAD_BPF_NEVER:
-		(void)0;
-		uint32_t prog_id = 0;
-		ret = bpf_get_link_xdp_id(iface->if_index, &prog_id, 0);
-		if (ret == 0 && prog_id != 0) {
-			ret = bpf_prog_get_fd_by_id(prog_id);
-		}
-		break;
 	case KNOT_XDP_LOAD_BPF_ALWAYS:
 		ret = ensure_prog(iface, true);
 		break;

src/libknot/xdp/xdp.h

@@ -44,7 +44,6 @@ struct knot_xdp_msg {
  *       a compatible BPF program is loaded on the interface.
  */
 typedef enum {
-	KNOT_XDP_LOAD_BPF_NEVER,  /*!< Do not load; error out if not loaded already. */
 	KNOT_XDP_LOAD_BPF_ALWAYS, /*!< Always load a program (overwrite it). */
 	KNOT_XDP_LOAD_BPF_MAYBE,  /*!< Try with present program or load if none. */
 	/* Implementation caveat: when re-using program in _MAYBE case, we get a message: