Remove trigger to deploy to release instance

Context

As described in &1571, the release instance is going to be deprecated.

In the release runbook, the release instance is mentioned in our release pipeline as below:

• "An Omnibus pipeline on dev is triggered as a result to build the package, Ubuntu-20.04-staging is the job that triggers the deploy to the release environment:" (note that "release environment" in this context means the release instance)

We need to remove this trigger

Exit Criteria

• Trigger to deploy to the release instance is removed from Omnibus pipeline
• Backport to all 3 supported version branches 18-9-stable , 18-8-stable and 18-7-stable