selftests/bpf: test_tunnel: Remove test_tunnel.sh

selftests/bpf: test_tunnel: Remove test_tunnel.sh

All tests from test_tunnel.sh have been migrated into test test_progs.
The last test remaining in the script is the test_ipip() that is already
covered in the test_prog framework by the NONE case of test_ipip_tunnel().

Remove the test_tunnel.sh script and its Makefile entry

Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250303-tunnels-v2-10-8329f38f0678@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>