index: remove postings_sketches

[dimitarvdimitrov]

Remove unused postings_sketches code. The Statistics interface and LabelsValuesSketches are only used in mimir where they're redefined.

Related to

• index planning: Implement per-block statistics mimir#12607
• index planning: define local Statistics interface mimir#13781

---

Note

Removes the postings sketches implementation and tests, and cleans up associated dependencies.

• Index/TSDB:
  • Remove tsdb/index/postings_sketches.go and tsdb/index/postings_sketches_test.go (count-min sketch–based Statistics implementation).
• Dependencies:
  • Drop github.com/tylertreat/BoomFilters and github.com/d4l3k/messagediff from go.mod; prune corresponding go.sum entries.

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