[NFC][TSan] Remove redundant Context::report_mtx (#228605) Context::report_mtx was only ever acquired immediately alongside ScopedErrorReportLock (in ScopedReport and BackgroundThread). Remove report_mtx and MutexTypeReport and rely on ScopedErrorReportLock. Assisted-by: Gemini GitOrigin-RevId: 04203a045b4cfb18e2b22764744249cba171eb30