Don’t show close icon on flash warning

What does this MR do?

Summary

Flash warnings show a close button, despite being in-dismissible.

Steps to reproduce

Sign up with a new account on GitLab.com. After signing up, a banner will be shown as in the screenshot.

What is the current bug behavior?

A close button is shown and clicking it does nothing.

What is the expected correct behavior?

The close button should not be shown.

Related to #34562 (closed)

Screenshots

before	after

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability 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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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