Remove incorrect docs from deploy_boards.md

What does this MR do?

We noticed this in https://gitlab.com/gitlab-org/gitlab-ee/issues/12006#note_180500023

This removes 2 incorrect sentences:

GitLab will then query Kubernetes for the state of each pod (e.g., waiting, deploying, finished, unknown), and the Deploy Board status will finally appear.

This sentence should have been removed when we made them expanded by default. In context it's not correct anymore.

GitLab will only display a Deploy Board for top-level environments. Foldered environments like review/* (usually used for [Review Apps]) won't have a Deploy Board attached to them.

From my testing deploy boards do display just fine for review apps:

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