Remove STUB_ATTRIBUTION_HMAC_KEY from mozorg demo env vars... (#12615)

...now it's in Secrets instead. With it present in both, Cloud Run fails
to boot the container

Co-authored-by: www-site-checker bot <None>

gcp/bedrock-demos/cloudrun/mozorg-demo.env.yaml

@@ -19,5 +19,4 @@ RUN_SUPERVISOR: "True"
 SECURE_SSL_REDIRECT: "True"
 SENTRY_DSN: https://97ec0cd426714b728e92f3b3aa62f00b@o1069899.ingest.sentry.io/6260338
 SITE_MODE: Mozorg
-STUB_ATTRIBUTION_HMAC_KEY: the-dude
 SWITCH_NEWSLETTER_MAINTENANCE_MODE: "False"