Remove unused audit-ci dependency

There is a `yarn ci` script which was using audit-ci but this does not
appear to be called anywhere.

The security worflow uses `yarn audit` and `npm audit` which seem fine
enough anyway.

docs/SECURITY.md

@@ -17,10 +17,8 @@ We use the following tools to help us stay on top of vulnerability mitigation.
     - Comprehensive vulnerability scanner that runs on PRs into the default
       branch and scans both our container image and repository code (see
       `trivy-scan-repo` and `trivy-scan-image` jobs in `build.yaml`)
-- [`audit-ci`](https://github.com/IBM/audit-ci)
-  - Audits npm and Yarn dependencies in CI (see `Audit for vulnerabilities` step
-    in `build.yaml`) on PRs into the default branch and fails CI if moderate or
-    higher vulnerabilities (see the `audit.sh` script) are present.
+- `yarn audit` and `npm audit`
+  - Audits Yarn/NPM dependencies.
 
 ## Supported Versions