selftests/bpf: Drop xdping tool

selftests/bpf: Drop xdping tool

As part of a larger cleanup effort in the bpf selftests directory,
tests and scripts are either being converted to the test_progs framework
(so they are executed automatically in bpf CI), or removed if not
relevant for such integration.

The test_xdping.sh script (with the associated xdping.c) acts as a RTT
measurement tool, by attaching two small xdp programs to two interfaces.
Converting this test to test_progs may not make much sense:
- RTT measurement does not really fit in the scope of a functional test,
  this is rather about measuring some performance level.
- there are other existing tests in test_progs that actively validate
  XDP features like program attachment, return value processing, packet
  modification, etc

Drop test_xdping.sh, the corresponding xdping.c userspace part, the
xdping_kern.c program, and the shared header, xdping.h

Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Paul Chaignon <paul.chaignon@gmail.com>
Link: https://patch.msgid.link/20260422-xdping-v2-2-c0f8ccedcf91@bootlin.com