Removes fallback warning for legacy deploy board labels

What does this MR do?

!14020 (merged) removed support for the old app label in GitLab 12.1, but added a warning instead if it failed to find any matching deployments but detected legacy deployments.

We now remove this warning, as it has been a major milestone, and people should have migrated since then

Screenshots

Does this MR meet the acceptance criteria?

Compatibility

header	Old FE	New FE
Old BE	As is ✅	Does not render anything
New BE	Does not show warning as API always returns false	Does not render anything

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.