Network still referenced from a VoltageLevel after removal

[sebalaig]

• Do you want to request a feature or report a bug?
  Bug

• What is the current behavior?
  Backport of fix related to Network still referenced from a VoltageLevel after removal powsybl-core#1960

• If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem

• What is the expected behavior?

• What is the motivation / use case for changing the behavior?

• Please tell us about your environment:

  • PowSyBl Version: ...
  • OS Version: ...

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, spectrum, etc)

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