Drop experiment_users table as it is no longer used

What does this MR do and why?

Dropping table as per this procedure - https://docs.gitlab.com/ee/development/migration_style_guide.html#dropping-a-database-table

Code already removed for it via !100862 (merged)

Checked for data impacts in #349246 (comment 812091121)

rollback verification: https://gitlab.com/gitlab-org/gitlab/-/jobs/3243468566

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)