Remove experiments model code

What does this MR do and why?

Deprecating/removing the model code as this is no longer needed in the product due to enhanced snowplow context tracking that has been implemented in the past year or so.

first step as per https://docs.gitlab.com/ee/development/migration_style_guide.html#dropping-a-database-table

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #349246 (closed)