bpf: Remove unused member rcu from bpf_struct_ops_map

The rcu member in bpf_struct_ops_map is not used after commit
b671c20 ("bpf: Retire the struct_ops map kvalue->refcnt.")

Remove it.

Suggested-by: Martin KaFai Lau <martin.lau@linux.dev>
Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
Link: https://lore.kernel.org/r/20241112145849.3436772-2-xukuohai@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Xu Kuohai

kernel/bpf/bpf_struct_ops.c

@@ -23,7 +23,6 @@ struct bpf_struct_ops_value {
 
 struct bpf_struct_ops_map {
 	struct bpf_map map;
-	struct rcu_head rcu;
 	const struct bpf_struct_ops_desc *st_ops_desc;
 	/* protect map_update */
 	struct mutex lock;