remove parents to use env prefix instead to avoid mixing both concepts

[JLugagne]

In the commit 3f4449054d043e278c83a837648d882823a0a3c7 the concept of parents was added to keep track of the ancestors but led to some regressions (cf: issue 1566).

Instead of adding this new concept, why not using the envPrefix which seems to be made for this purpose? It may change Sub's behavior but makes it more intuitive I believe. And keeps the behavior defined by the TestEnvSubConfig.

[CLAassistant]

All committers have signed the CLA.

[github-actions]

👋 Thanks for contributing to Viper! You are awesome! 🎉

A maintainer will take a look at your pull request shortly. 👀

In the meantime: We are working on Viper v2 and we would love to hear your thoughts about what you like or don't like about Viper, so we can improve or fix those issues.

⏰ If you have a couple minutes, please take some time and share your thoughts: https://forms.gle/R6faU74qPRPAzchZ9

📣 If you've already given us your feedback, you can still help by spreading the news,
either by sharing the above link or telling people about this on Twitter:

https://twitter.com/sagikazarmark/status/1306904078967074816

Thank you! ❤️

[robdesbois]

@TaylorOno @sagikazarmark you co-authored the original commit - any comments on this resolution to #1566?