Delete vue_issuables_list feature flag

What does this MR do and why?

This MR deletes the vue_issuables_list feature flag as it has been superceded by the vue_issues_list feature flag.

This MR also updates the service desk empty state text. The text was updated in !40454 (diffs) for the vue_issuables_list refactor but the Haml template wasn't updated simultaneously, so the Haml template is updated here to satisfy the spec/features/issues/service_desk_spec.rb specs.

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