Investigate if `GITLAB_QA_ISSUE_URL` is required for QA canary jobs

While implementing #1490 (closed) it was noticed that all QA jobs for gprd-cny sent GITLAB_QA_ISSUE_URL as an empty value. Example:

2021-11-03 09:40:24.054293 I Trigger::Qa -- Triggered downstream pipeline -- {:web_url=>"https://ops.gitlab.net/gitlab-org/quality/canary/-/pipelines/872534", :id=>872534, :project=>"gitlab-org/quality/canary", :variables=>["GITLAB_QA_ISSUE_URL=", "DEPLOY_ENVIRONMENT=gprd-cny", "DEPLOY_VERSION=14.5.202111030620-c1ef75cba80.fb7ecd0d5b4", "SMOKE_ONLY"]}

We mirrored the same behavior in the coordinated pipeline, but we should investigate if it's required, and if not, remove it from the variables in .gitlab-ci.yml.