fix regression in approval-voting introduced in #3747 (#3831)

Fixes #3826.

The docs on the `candidates` field of `BlockEntry` were incorrectly
stating that they are sorted by core index. The (incorrect) optimization
was introduced in #3747 based on this assumption. The actual ordering is
based on `CandidateIncluded` events ordering in the runtime. We revert
this optimization here.

- [x] verify the underlying issue
- [x] add a regression test

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Co-authored-by: Bastian Köcher <[email protected]>