Removes legacy GlobalLabel class

What does this MR do?

Removes the last usage of GlobalLabel. This changes the dashboard labels endpoint to get a unique list of label titles using SQL.

GlobalLabel used to be how we implemented group labels where we grouped project labels by title.

For this dashboard endpoint, we're not showing group labels but we're only returning labels that have unique titles because these are used by the filter bar and we filter by title so duplicates are redundant.

Related to #11265 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
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• Style guides
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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
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