CI: audit: remove ignore of time 0.1 vuln (no longer present)

this is effectively a `git revert 15acb6d` (but not done as such to
provide a more detailed commit message).
starting with chrono v0.4.30 the dependency to `time` has been removed,
accordingly `cargo audit` no longer finds any issues here.

.github/workflows/CI.yml

@@ -39,7 +39,7 @@ jobs:
       - name: check formatting
         run: cargo fmt --all -- --check
       - name: audit
-        run: cargo audit --ignore RUSTSEC-2020-0071 # time 0.1, pulled in through chrono which however doesn't use the affected API. see https://github.com/chronotope/chrono/issues/602
+        run: cargo audit
 
   clippy:
     runs-on: ubuntu-latest