selftests/bpf: Remove unused variable in tc_tunnel prog

The variable buf is unused since commit 005edd16 ("selftests/bpf:
convert bpf tunnel test to BPF_ADJ_ROOM_MAC"). Remove it to fix the
following warning:

    test_tc_tunnel.c:531:7: warning: unused variable 'buf' [-Wunused-variable]

Fixes: 005edd16 ("selftests/bpf: convert bpf tunnel test to BPF_ADJ_ROOM_MAC")
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Willem de Bruijn <willemb@google.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20210713102719.8890-1-tklauser@distanz.ch

tools/testing/selftests/bpf/progs/test_tc_tunnel.c

@@ -528,7 +528,6 @@ int __encap_ip6vxlan_eth(struct __sk_buff *skb)
 
 static int decap_internal(struct __sk_buff *skb, int off, int len, char proto)
 {
-	char buf[sizeof(struct v6hdr)];
 	struct gre_hdr greh;
 	struct udphdr udph;
 	int olen = len;