bpf: Drop useless btf_vmlinux in bpf_tcp_ca

The code using btf_vmlinux in bpf_tcp_ca is removed by the
commit 9f0265e9 ("bpf: Require only one of cong_avoid() and cong_control() from a TCP CC")
so drop this useless btf_vmlinux declaration.
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Link: https://lore.kernel.org/r/4d38da4eadaba476bd92ffcd7a5a03a5e28745c0.1689582557.git.geliang.tang@suse.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

net/ipv4/bpf_tcp_ca.c

@@ -51,8 +51,6 @@ static bool is_unsupported(u32 member_offset)
 	return false;
 }
 
-extern struct btf *btf_vmlinux;
-
 static bool bpf_tcp_ca_is_valid_access(int off, int size,
 				       enum bpf_access_type type,
 				       const struct bpf_prog *prog,