Remove legacy epic to work items sync spec

Now that they're no longer used, we need to remove the legacy epic to work items sync spec and corresponding scheduled pipeline.

• Delete scheduled pipeline
• Archive associated test cases
• Delete sync spec qa/qa/specs/features/ee/api/2_plan/epics_to_work_items_sync_spec.rb gitlab-org/gitlab!207679 (merged)
• Remove EPIC_SYNC_TEST variable from gitlab qa gitlab-org/gitlab-qa!1435 (merged)
• Remove Epic migration test job from pipeline-common gitlab-org/quality/pipeline-common!542 (merged)
• Consolidate work_item_epic into epic resource for e2e specs