Remove merge gatekeeper

[westey-m]

Motivation and Context

Github should block on any failed job as it is, so it's unclear what merge gatekeeper does.

Description

• Removing merge gatekeeper job.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.