Remove the X-Yst-Cohort header so Yoast can use it for other purposes

[wpscholar]

Proposed changes

Remove the X-Yst-Cohort header so Yoast can use it for other purposes.

Wait to release this until after March 4th

Type of Change

Production

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Dependency update
• Refactoring / housekeeping (changes to files not directly related to functionality)

Development

• Tests
• Dependency update
• Environment update / refactoring
• Documentation Update

Checklist

• I have read the CONTRIBUTING doc
• I have viewed my change in a web-browser
• Linting and tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)