selftests/bpf: Fix an unused-but-set-variable compiler warning

When using clang to build selftests with LLVM=1 in make commandline,
I hit the following compiler warning:
  xdpxceiver.c:747:6: warning: variable 'total' set but not used [-Wunused-but-set-variable]
          u32 total = 0;
              ^

This patch fixed the issue by removing that declaration and its
assocatied unused operation.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211112204833.3579457-1-yhs@fb.com