Remove unused field for Cohere chat calls

[acatav]

Problem

The preamble_override field for cohere chat completion changed to preamble, but we don't actually use it

Solution

Remove the use of preamble

Type of Change

• Bug fix (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)
• This change requires a documentation update
• Infrastructure change (CI configs, etc)
• Non-code change (docs, etc)
• None of the above: (explain here)

Test Plan

Existing tests apply