use: git remote prune origin
or use git remote prune origin --dry-run to preview what branches will be removed.
As in git help remote
prune
Deletes all stale remote-tracking branches under . These stale branches have already been removed from the remote repository referenced by , but are still locally available in “remotes/”.
With –dry-run option, report what branches will be pruned, but do not actually prune them.