selftests/bpf: Drop netns helpers in mptcp

New netns selftest helpers netns_new() and netns_free() has been added
in network_helpers.c, let's use them in mptcp selftests too instead of
using MPTCP's own helpers create_netns() and cleanup_netns().

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/c02fda3177b34f9e74a044833fda9761627f4d07.1730338692.git.tanggeliang@kylinos.cn


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>