Remove mr_throughput_clickhouse_data_source feature flag

What does this MR do and why?

Removes the mr_throughput_clickhouse_data_source feature flag, which gated reading Merge Request throughput from ClickHouse on the Value Streams Dashboard and the AI-impact / Duo & SDLC trends dashboards. The flag was enabled globally on GitLab.com on 2026-09-19 and has soaked cleanly since. This deletes the flag definition, drops the flag check from merge_request_throughput_data_source_clickhouse? (the ClickHouse source now depends only on ClickHouse-for-analytics availability plus the SaaS Siphon feature), and removes the three dead "flag disabled" spec contexts.

Rollout: #627313 (closed). Original bug: https://gitlab.com/gitlab-org/gitlab/-/work_items/627208.

Feature flag

mr_throughput_clickhouse_data_source was fully rolled out on GitLab.com. This MR removes it.

Validation

Soaked from the 2026-09-19 global enable through 2026-09-24, checked every few hours (soak log). Results:

• Zero errors on the ClickHouse path (graphql:mergeRequestsThroughputClickhouse) across ~3,200 requests.
• Zero QueryCanceled timeouts on the old path fleet-wide since the enable. Baseline before the fix was around 45 per week.
• New-path latency held steady as traffic grew past 1,200 requests per day: p50 340ms, p95 630ms, p99 1.0s.

One behaviour to keep in mind after removal: the ClickHouse source still falls back to the Postgres count when enabled_for_analytics? is false for a group, so a group with ClickHouse analytics disabled stays on the slower path. Tracked as a follow-up on the rollout issue.