nanobench: Stop reporting median_ms, it's no better than min_ms.

BUG=skia:
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Review URL: https://codereview.chromium.org/1825473002

bench/nanobench.cpp

@@ -1200,7 +1200,6 @@ int nanobench_main() {
             benchStream.fillCurrentOptions(log.get());
             target->fillOptions(log.get());
             log->metric("min_ms",    stats.min);
-            log->metric("median_ms", stats.median);
 #if SK_SUPPORT_GPU
             if (gpuStatsDump) {
                 // dump to json, only SKPBench currently returns valid keys / values