samples/bpf: remove unused function with test_lru_dist

samples/bpf: remove unused function with test_lru_dist

Currently, compiling samples/bpf with LLVM warns about the unused
function with test_lru_dist.

    ./samples/bpf/test_lru_dist.c:45:19:
    warning: unused function 'list_empty' [-Wunused-function]
    static inline int list_empty(const struct list_head *head)
                      ^
                      1 warning generated.

This commit resolve this compiler warning by removing the abandoned
function.

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221218061453.6287-2-danieltimlee@gmail.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>