test-network: do not pass '[detached]' to 'ip rule del'

test-network: do not pass '[detached]' to 'ip rule del'

That indicates the interface name in 'iif' or 'oif' cannot be resolved
when 'ip rule' command is invoked. That's natural when networkd fail to
remove rule but the corresponding interface is already removed.
To make not the residual rules interfere subsequent test cases, let's
ignore the flag and actually remove unwanted rules.