chore: cleanup un-wanted region tags (GoogleCloudPlatform#1787)

Description: Old region tags are replaced with product specific region tags. Please check b/272509882 for more details.

New Region tags were added using Pull Request: https://github.com/GoogleCloudPlatform/php-docs-samples/pull/1784/files

Author: msampathkumar

cdn/signUrl.php

@@ -15,7 +15,6 @@
  * limitations under the License.
  */
 
-# [START signed_url]
 # [START cloudcdn_sign_url]
 /**
  * Decodes base64url (RFC4648 Section 5) string
@@ -83,4 +82,3 @@ function sign_url(http://github.com/$url,%20$keyName,%20$base64UrlKey,%20$expirationTime)
     return "{$url}&Signature={$encodedSignature}";
 }
 # [END cloudcdn_sign_url]
-# [END signed_url]