Migrate hook test dropdown to new GlDropdownDisclosure API

In !109534 (merged) the hook test dropdown was migrated to GlDropdownDisclosure, but had to implement a hack to work around some limitations with the component.

With gitlab-ui!3275 (merged), those limitations should be removed, so the implementation of !109534 (merged) can be simplified.

Implementation plan

• Remove hack.
• Rely purely on href and extraAttrs in items to set data-method attribute.