Remove gl-dropdown-item-deprecated-adapter class

What does this MR do?

This MR removes a now unnecessary gl-dropdown-item-deprecated-adapter class. This class was made unnecessary by the @gitlab/ui MR gitlab-ui!1969 (merged).

Screenshots (strongly suggested)

This MR actually removes us overwriting the secondary text color so there's a slight change here:

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