Pr/remove rabitq reorder prefilter

[xtangxtang]

PR: Remove RaBitQ reorder-prefilter + README cleanup

What changed

This PR removes the previously added RaBitQ reorder prefilter feature end-to-end, while keeping RaBitQ main-search approximate scoring support.

DiskANN core (SSD search)

• Removed all reorder-prefilter state and logic from PQFlashIndex.
  • No longer loads/uses <index>_rabitq_reorder.bin.
  • No longer supports DISKANN_USE_RABITQ_REORDER_PREFILTER / related tuning env vars.
• Kept RaBitQ main traversal approximate scorer support (runtime-gated).

Build / CLI integration

• Removed build-time generation options and plumbing for reorder-prefilter codes.
• Removed the reorder-code generator app from the build targets.

Documentation

• Removed the entire “RaBitQ multi-bit reorder prefilter” section from the top-level README.
• Updated the RaBitQ main-search README section to remove the old fallback wording that referenced reorder-code sidecars.

Why

The reorder-prefilter mode is no longer desired; maintaining only the RaBitQ main-search mode simplifies the feature surface and avoids documenting/maintaining a removed behavior.

Notes

• RaBitQ main-search mode remains runtime-gated (disabled by default).
• If you have existing indexes/code sidecars produced for reorder-prefilter, they are no longer consumed by the library.

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