Ifdef out WarnIfComponentBlockingGuard for benchmark builds

Add USE_BENCHMARK define to benchmark build flags. Guard the
warn_blocking call in finish() with #ifndef USE_BENCHMARK so
scheduler benchmarks using fake monotonic time don't trigger
the underflow (fake now > real millis()).

Remove BenchComponent — no longer needed with the ifdef.

script/cpp_benchmark.py

@@ -34,6 +34,7 @@ PLATFORMIO_OPTIONS = {
         "-O2",  # optimize for speed (CodSpeed recommends RelWithDebInfo)
         "-g",  # debug symbols for profiling
         USE_TIME_TIMEZONE_FLAG,
+        "-DUSE_BENCHMARK",  # disable WarnIfComponentBlockingGuard in finish()
     ],
     # Use deep+ LDF mode to ensure PlatformIO detects the benchmark
     # library dependency from nested includes.