samples/bpf: Remove unused variable

The variable is never referenced in the code, just remove it
that this problem was discovered by reading code

Signed-off-by: Zhu Jun <zhujun2@cmss.chinamobile.com>
Link: https://lore.kernel.org/r/20241120032241.5657-1-zhujun2@cmss.chinamobile.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>