chore: rm unused cosign step

https://github.com/bitwarden/key-connector/pull/239

Remove installation of cosign step as part of publish job

.github/workflows/publish.yml

@@ -55,9 +55,6 @@ jobs:
       id-token: write
       packages: write
     steps:
-      - name: Install Cosign
-        uses: sigstore/cosign-installer@faadad0cce49287aee09b3a48701e75088a2c6ad # v4.0.0
-
       - name: Login to GitHub Container Registry
         uses: docker/login-action@184bdaa0721073962dff0199f1fb9940f07167d1 # v3.5.0
         with: