bpftool: Remove unused struct event_ring_info

After commit 9b190f18

 ("tools/bpftool: switch map event_pipe to
libbpf's perf_buffer"), struct event_ring_info is not used any more and
can be removed as well.

Signed-off-by: Yuan Can <yuancan@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220928090440.79637-3-yuancan@huawei.com