Remove web_ide_oauth flag

What does this MR do and why?

The web_ide_oauth feature flag is ready to remove as this feature has been tested thoroughly and we have not received bug reports from self- managed customers post enabling the flag by default. In this MR:

• The flag definition and all references of the flag are removed
• Updates docs that refer to this flag in its history

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

New OAuth application	With existing OAuth application
new_app.mov	demo.mov

How to set up and validate locally

New application:

• In the Web IDE, remove existing tokens in local storage by removing items with the following key: gitlab/web-ide/oauth/<application_id>/token.
• In Admin Area > Applications, remove the GitLab Web IDE application.
• Refresh the Web IDE:
  • Observe OAuth redirect process
  • In Admin Area > Applications, a new GitLab Web IDE application is created