selftests/bpf: Drop tcp-{client,server}.py from Makefile

The files don't exist anymore so this breaks generic kselftest builds
when using "make install" or "make gen_tar".

Fixes: 247f0ec3 ("selftests/bpf: Drop python client/server in favor of threads")
Signed-off-by: Veronika Kabatova <vkabatov@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20201210120134.2148482-1-vkabatov@redhat.com