Remove association between ChatName and Integration - Step 3

What does this MR do and why?

As a part of Identify users in Slack without a project or integration epic, and per issue, this MR removes:

• integration_id column from the chat_names table
• Project and Service values from active chat names preview /-/profile/chat_names
• relationship between integrations and chat_names
• irrelevant specs

Screenshots or screen recordings

Before	Now

How to set up and validate locally

1. Checkout the bmarjanovic/remove-association-between-chatname-and-integration-step-3 branch
2. Run rails db:migrate command
3. Visit the chat_names page, and validate there are no Project and Service values (e.g. http://gdk.test:3000/-/profile/chat_names)

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 #384974 (closed)