libbpf: Remove sample_period init in perf_buffer

libbpf: Remove sample_period init in perf_buffer

It seems that sample_period is not used in perf buffer. Actually, only
wakeup_events are meaningful to enable events aggregation for wakeup notification.
Remove sample_period setting code to avoid confusion.

Fixes: fb84b8224655 ("libbpf: add perf buffer API")
Signed-off-by: Tao Chen <chen.dylane@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/bpf/20250423163901.2983689-1-chen.dylane@linux.dev