nmci/cleanup.py: subtle bugs

Playing with type hints in cleanup.py (in order to get rid of warnings emitted by sphinx-build -n) revealed these little bugs...

Also, CleanupNMConfig seems unused so far.

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@RunTests:ipv4_mptcp_no_flags_no_defroute,ipv4_mptcp_no_flags,ipv4_mptcp_flags_enabled,ipv4_mptcp_flags_0x2_0x4,ipv4_mptcp_flags_0x10,ipv4_mptcp_flags_0x20,ipv4_mptcp_flags_0x40,ipv4_mptcp_flags_0x80,ipv4_mptcp_reapply_change_flag,ipv4_mptcp_remove_endpoints,ipv6_mptcp_no_flags,ipv6_mptcp_flags_0x8