selftests/bpf: Remove unused variable from bpf_sk_assign test

Was never used in bpf_sk_assign_test(), and was removed from handle_{tcp,udp}()
in commit 0b9ad56b ("selftests/bpf: Use SOCKMAP for server sockets in
bpf_sk_assign test").

Fixes: 0b9ad56b ("selftests/bpf: Use SOCKMAP for server sockets in bpf_sk_assign test")
Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220329154914.3718658-1-eyal.birger@gmail.com