bpf: split shared bpf_tcp_sock and bpf_sock_ops implementation

We've added bpf_tcp_sock member to bpf_sock_ops and don't expect
any new tcp_sock fields in bpf_sock_ops. Let's remove
CONVERT_COMMON_TCP_SOCK_FIELDS so bpf_tcp_sock can be independently
extended.

Cc: Eric Dumazet <edumazet@google.com>
Cc: Priyaranjan Jha <priyarjha@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>