refactor: remove dead getGlobalState("taskHistory") fallback reads #1543

Part of #1541. Cleanup after #1542.

After the write-through is removed, the three getGlobalState("taskHistory") fallbacks become unreachable dead code. This PR removes them.

Changes

ClineProvider.ts only — three call sites:

• Line 1988: this.taskHistoryStore.get(task.taskId) ?? (this.getGlobalState("taskHistory") ?? []).find(...) → this.taskHistoryStore.get(task.taskId)
• Line 2251: same pattern
• Line 2521: same pattern

Safety

Safe to merge standalone even before #1542 — removing the fallback just means the globalState path is exercised sooner rather than later. No behaviour change while globalState still holds data.