Remove unreachable code from config._prepareconfig

[phloose]

Related to #4445 (comment) and #4445 (comment). The code that could trigger the execution of _issue_warning_captured was removed with a7e4016.

• Target the master branch for bug fixes, documentation updates and trivial changes.
• Target the features branch for new features, improvements, and removals/deprecations.
• Include documentation when adding new features.
• Include new tests or update existing tests when applicable.

Unless your change is trivial or a small documentation fix (e.g., a typo or reword of a small section) please:

• Create a new changelog file in the changelog folder, with a name like <ISSUE NUMBER>.<TYPE>.rst. See changelog/README.rst for details.
• Add yourself to AUTHORS in alphabetical order;

[nicoddemus]

Thanks for this!