Remove CPU telemetry (background temperature/clock speed sampling)

The background thread that sampled CPU temperature and clock speed
during a run relied on undocumented, OS-specific interfaces: raw
AppleSMC keys on macOS, WMI ACPI thermal zones on Windows, and
/sys/class/thermal or /sys/class/hwmon on Linux. Testing on Apple
Silicon showed the SMC key scan can silently switch between different
physical sensors mid-run, producing discontinuous jumps in reported
temperature, and live per-core clock speed was never obtainable on
macOS through any public API. Disable the feature entirely rather
than ship unreliable data: drop the telemetry monitor thread, its
platform-specific sensor backends, and the telemetry field from
upload payloads. The pre-run background system-activity check
(--no-system-check) is unrelated and untouched.

Bump FB_VERSION to 0.4.2.

.github/workflows/release.yml

@@ -175,5 +175,5 @@ jobs:
             release/SHA256SUMS \
             --repo "$GITHUB_REPOSITORY" \
             --title "Release $RELEASE_TAG" \
-            --notes "Hotfix: CPU telemetry (temperature and clock speed) recorded during benchmark runs was not working on Windows or macOS, showing empty temperature data points and a constant clock speed. Windows temperature now reads the ACPI thermal zone over WMI, and clock speed reads live per-core frequency via CallNtPowerInformation instead of a static registry value. macOS temperature now reads the SMC directly across Intel and Apple Silicon (M1-M5) Macs. macOS clock speed now reports as unavailable rather than the fixed nominal frequency it previously showed, since no live-frequency API exists on macOS." \
+            --notes "Removed CPU telemetry (background temperature and clock speed sampling during benchmark runs). It relied on undocumented, OS-specific interfaces (raw AppleSMC keys on macOS, WMI ACPI thermal zones on Windows, /sys/class/thermal and /sys/class/hwmon on Linux) that proved unstable and inconsistent across operating systems and hardware, and live per-core clock speed was never obtainable on macOS at all through any public API. Rather than ship telemetry data that can silently jump between physical sensors mid-run or simply not exist on a given platform, this release disables the feature entirely: fossbench.net no longer accepts or displays it, and the client no longer collects it. The one-time background system-activity check (CPU/memory/process sampling before a run) is unaffected and still available via --no-system-check." \
             --generate-notes