ci: delete inactive kokoro release folder

Deleted .kokoro/release as there are no recent job runs in the cloud-devrel namespace. This folder contained usage of yoshi-automation-github-key.

b/510338703

TAG=agy

Author: suztomo

.kokoro/docs/common.cfg

@@ -47,16 +47,6 @@ env_vars: {
     value: ".kokoro/docker/docs/Dockerfile"
 }
 
-# Fetch the token needed for reporting release status to GitHub
-before_action {
-  fetch_keystore {
-    keystore_resource {
-      keystore_config_id: 73713
-      keyname: "yoshi-automation-github-key"
-    }
-  }
-}
-
 before_action {
   fetch_keystore {
     keystore_resource {