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

tools/bpf/bpftool/map_perf_ring.c

@@ -29,13 +29,6 @@
 
 static volatile bool stop;
 
-struct event_ring_info {
-	int fd;
-	int key;
-	unsigned int cpu;
-	void *mem;
-};
-
 struct perf_event_sample {
 	struct perf_event_header header;
 	__u64 time;