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

sdmx_datasets_ecb/meta.json

@@ -24,6 +24,5 @@
     "builder": "reshape_datasets.py (datasets-catalog reshape)",
     "content_sha256": "97048ab6a4a00fd331c47fca0edfb25dd9178d60813e251895efc619d3505705",
     "manifest_contract_sha256": "c486deb4ac507c701c20c0e7dd4125e04fe722b25ecb3a2d97265e4f2886468a"
-  },
-  "sdmx": null
+  }
 }