selftests: net: lib: remove ns from list after clean-up

Instead of only appending items to the list, removing them when the
netns has been deleted.

By doing that, we can make sure 'cleanup_all_ns()' is not trying to
remove already deleted netns.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240607-upstream-net-next-20240607-selftests-mptcp-net-lib-v1-2-e36986faac94@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>