Corrected behavior of uninstall --all

[matteius]

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The issue

When I recently refactored the uninstall command behavior to be smarter, I broke the expected behavior of --all flag. Specifically there should not be removal from the Pipfile and the do_purge logic should be invoked (it wasn't after my change). Fixes #6200

The checklist

• [X] Associated issue
• [X] A news fragment in the news/ directory to describe this fix with the extension .bugfix.rst, .feature.rst, .behavior.rst, .doc.rst. .vendor.rst. or .trivial.rst (this will appear in the release changelog). Use semantic line breaks and name the file after the issue number or the PR #.