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