Removes unknown team from codeowners file, removes duplicate

.github/CODEOWNERS

@@ -3312,7 +3312,6 @@ x-pack/solutions/observability/plugins/observability @elastic/obs-ux-management-
 x-pack/solutions/observability/plugins/observability_ai_assistant_app @elastic/obs-ai-assistant
 x-pack/solutions/observability/plugins/observability_ai_assistant_management @elastic/obs-ai-assistant
 x-pack/solutions/observability/plugins/observability_logs_explorer @elastic/obs-ux-logs-team
-x-pack/solutions/observability/plugins/observability_onboarding @elastic/obs-ux-logs-team @elastic/obs-ux-onboarding-team
 x-pack/solutions/observability/plugins/observability_shared @elastic/observability-ui
 x-pack/solutions/observability/plugins/profiling @elastic/obs-ux-infra_services-team
 x-pack/solutions/observability/plugins/profiling_data_access @elastic/obs-ux-infra_services-team