revert cosign signing of release checksums file (#2571)

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

.goreleaser.yaml

@@ -259,16 +259,3 @@ sboms:
       - "$artifact"
       - "--output"
       - "json=$document"
-
-signs:
-  - cmd: .tool/cosign
-    signature: "${artifact}.sig"
-    certificate: "${artifact}.pem"
-    args:
-      - "sign-blob"
-      - "--oidc-issuer=https://token.actions.githubusercontent.com"
-      - "--output-certificate=${certificate}"
-      - "--output-signature=${signature}"
-      - "${artifact}"
-      - "--yes"
-    artifacts: checksum