Remove duplicate PyPIRepository().finder.find_all_candidates cache

[gschaffner]

Hi! I believe that PyPIRepository()._available_candidates_cache is redundant, as PackageFinder().find_all_candidates has been lru_cached since Pip 20.3 (https://github.com/pypa/pip/commit/1dd6d562789b06dc1508212505e32f36bd71d310).

Contributor checklist

• [ ] Provided the tests for the changes.
• [x] Assure PR title is short, clear, and good to be included in the user-oriented changelog

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• [ ] Assure one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
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