[40network] Do not delete finished signal in net_add_parse_vars' finished hook

In case we are not the only one not being finished we could not detect being finished when we're run the next time.
Since dracut does not delete finished hooks which succeeded, we will get stuck.
Merge branch 'feature/network'

* feature/network:
  [40network] Do not delete finished signal in net_add_parse_vars' finished hook