Remove `CHANGELOG.md` from the PR template

[bessbd]

Checklist

• [x] Tick to sign-off your agreement to the Developer Certificate of Origin (DCO) 1.1
• [x] Added tests for code changes or test/build only changes
• [x] Updated the change log file (CHANGES.md|CHANGELOG.md) or test/build only changes
• [x] Completed the PR template below:

Description

This repo only has a CHANGES.md and CHANGELOG.md is not applicable. This PR removes the CHANGELOG.md reference from the template.

Approach

Schema & API Changes

Security and Privacy

Testing

Monitoring and Logging

[bessbd]

Thank you for the review, @ricellis ! I'm about to merge this.