Remove sign_blob_test.sh test

The bug that this test was meant to address[1] applies to a feature that
was removed from cosign[2]. The updates made to the script to allow for
the breaking changes actually make the test invalid, because while it
was once verifying the original artifact signature, the final
verification step is now actually verifying against the uploaded rekor
entry, so the original signature is inconsequential.

[1] #1673
[2] #2425

Signed-off-by: Colleen Murphy <colleenmurphy@google.com>

.github/workflows/github-oidc.yaml

@@ -62,8 +62,3 @@ jobs:
           set -e
           # Build and publish an image.
           make sign-ci-keyless-containers
-
-      - name: Build and sign a blob
-        run: |
-          set -e
-          make sign-blob-experimental

test/ci.mk

@@ -9,7 +9,3 @@ sign-ci-containers: ko
 .PHONY: sign-ci-keyless-containers
 sign-ci-keyless-containers: ko
 	./scripts/sign-images-ci.sh
-
-.PHONY: sign-blob-experimental
-sign-blob-experimental:
-	./test/sign_blob_test.sh