Remove setImmediate polyfill from production assets

What does this MR do?

Removes the setImmediate polyfill from our frontend code. This is automatically injected by webpack, and was causing confusion by tripping up security scanners. While there is not actually a security vulnerability to be found in the setImmediate polyfill, the polyfill itself appears to be completely superfluous anyway, so this MR aims to remove it and avoid future confusion.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Part of: #64403 (moved)