selftests/bpf: Remove test_xdp_redirect_multi.sh

selftests/bpf: Remove test_xdp_redirect_multi.sh

The tests done by test_xdp_redirect_multi.sh are now fully covered by
the CI through test_xdp_veth.c.

Remove test_xdp_redirect_multi.sh
Remove xdp_redirect_multi.c that was used by the script to load and
attach the BPF programs.
Remove their entries in the Makefile

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/20250212-redirect-multi-v5-6-fd0d39fca6e6@bootlin.com