Remove deprecated method ClusterEnvironment.creates_children

[awaelchli]

What does this PR do?

Part of #10312
Follow up to #10106

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

Anyone in the community is free to review the PR once the tests have passed.
Before you start reviewing make sure you have read Review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

Did you have fun?

I made sure I had fun coding 🙃

Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

[ananthsub]

should we have a github label for cluster environments?

[four4fish]

qq: we can start remove 1.6 removable items right after 1.5 release?

[awaelchli]

Is your question about the short deprecation time of this method?
Normally we have a two version jump, e.g., from 1.5 to 1.7. Here we did only one so that's why I'm removing it already.
If it's a concern, we can also hold off one more release and increase the deprecation one version.

[codecov]

Codecov Report

Merging #10339 (325a5d4) into master (c5d011c) will decrease coverage by 0%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #10339   +/-   ##
=======================================
- Coverage      93%      93%   -0%     
=======================================
  Files         181      181           
  Lines       16496    16492    -4     
=======================================
- Hits        15270    15266    -4     
  Misses       1226     1226