sparse registry has been stable for a bit now

Author: RalfJung

src/tools/miri/.github/workflows/ci.yml

@@ -12,9 +12,6 @@ on:
   schedule:
     - cron: '11 5 * * *' # At 5:11 UTC every day.
 
-env:
-  CARGO_UNSTABLE_SPARSE_REGISTRY: 'true'
-
 defaults:
   run:
     shell: bash