bpf: Remove repeated struct btf_type declaration

struct btf_type is declared twice. One is declared at 35th line. The below
one is not needed, hence remove the duplicate.
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20210401072037.995849-1-wanjiabing@vivo.com

include/linux/bpf.h

@@ -928,7 +928,6 @@ struct bpf_link_primer {
 };
 
 struct bpf_struct_ops_value;
-struct btf_type;
 struct btf_member;
 
 #define BPF_STRUCT_OPS_MAX_NR_MEMBERS 64