Remove the native styles for modal-dialog - Currently off center

What does this MR do?

Fixes the top and bottom margins of our modal-dialog, currently it's significantly off center.

With gitlab-ui!1717 (merged) we added styles to center the modal, which included a x and y margin style. Currently on gitlab.com, we are setting the modal-dialog margin styles, which should be removed in favor the latest css changes in gitlab-ui so our modals are centered as intended.

Screenshots (strongly suggested)

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