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.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>