[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
4 files changed