[GLUTEN-13341][VL] Remove the unused veloxToArrow metric

[LuciferYang]

What changes are proposed in this pull request?

The native veloxToArrow metric, the time ensureFlattened spends flattening a batch before export, is sent to the JVM in Metrics.SingleMetric, but nothing has read it since #3854. This removes the metric and the code that carries it:

• the timer in VeloxColumnarBatch::ensureFlattened and BoltColumnarBatch::ensureFlattened, and ColumnarBatch::getExportNanos
• ResultIterator::setExportNanos and getExportNanos, whose field was never initialized
• the exportNanos parameter of Runtime::getMetrics and WholeStageResultIterator::getMetrics
• the veloxToArrow argument of the JNI Metrics constructor, and Metrics.SingleMetric
• the SingleMetric parameter of MetricsUtil.updateTransformerMetricsInternal and JoinMetricsUpdater.updateJoinMetrics, which only passed it along

The Velox and Bolt backends share cpp/core and the JNI constructor, so both are changed. ClickHouse has its own NativeMetrics class and is not affected.

How was this patch tested?

No new tests. The metrics JSON the JVM reads is unchanged. CI does not build the Bolt C++ code, and those changes were not compiled. The Bolt JVM module compiles.

Was this patch authored or co-authored using generative AI tooling?

Yes. Generated-by: Claude Opus.

Related issue: #13341.

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