Remove internal upload-preparation marker

The public release is complete; this internal pre-upload status file is no longer part of the dataset.

READY_TO_UPLOAD.json

@@ -1,9 +0,0 @@
-{
-  "archive_count": 10,
-  "checksum_entry_count": 41,
-  "dataset_total_pairs": 5000,
-  "passed": true,
-  "prepared_at_utc": "2026-08-18T15:54:55.107905+00:00",
-  "release_manifest_sha256": "427ff5d0b3ca4fad6ca075efd0dae845728e5ef41526b5fe02e822380fb6cf1e",
-  "source_artifact_content_sha256": "f51ba91e7da337518c4f0c00738157a1d51bb22ce094ff15ce7e6de8b4a1eea4"
-}