fix(sdmx_datasets_estat): strip stale sdmx key from meta.json (post-#82 manifest contract)

sdmx_datasets_estat/meta.json

@@ -24,6 +24,5 @@
     "builder": "reshape_datasets.py (datasets-catalog reshape)",
     "content_sha256": "4764aa58a1be548ea120e7dc636467bd227e0d3e994820ec071d2697b93874ac",
     "manifest_contract_sha256": "019d6719a9db4bd499de152683ec5cd6baa5b79c77efe33b8865ce95e09491bb"
-  },
-  "sdmx": null
+  }
 }