selftests/bpf: Remove serial from tests using {open,close}_netns

After removing the mount/umount dance from {open,close}_netns()
in the pervious patch, "serial_" can be removed from
the tests using {open,close}_netns().

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20221129070900.3142427-7-martin.lau@linux.dev