Remove configFlagError

[a-02]

This commit removes the configFlagError field in ConfigFlags.

This flag was never implemented and is currently not used by any internal or external function. Unless I'm missing some convention against removing flags, this shouldn't exist.

Gimme a second to add in the stuff for template A.

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Template Α: This PR modifies behaviour or interface

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
  • Is the change significant? If so, remember to add significance: significant in the changelog file.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added. (Ask for help if you don’t know how to write them! Ask for an exemption if tests are too complex for too little coverage!)

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Template B: This PR does not modify behaviour or interface

E.g. the PR only touches documentation or tests, does refactorings, etc.

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

[telser]

@a-02 Looks like there is a test that needs to be adjusted. But in general I'm highly in favor of removing items that are not usable in any way.