fix: Deephaven express memory leak (#277)

Fixes #179 

Tested by running a server with and without this fix and opening the
Memory dev tools tab. Then ran the code in the ticket. The tab without
this fix grew steadily in memory consumption over several minutes. The
tab with the fix stayed fairly consistent over the same period of time.

plugins/plotly-express/src/js/src/PlotlyExpressChartModel.ts

@@ -241,7 +241,7 @@ export class PlotlyExpressChartModel extends ChartModel {
     figureUpdateEvent.columns.forEach(column => {
       const columnData = chartData.getColumn(
         column.name,
-        val => this.chartUtils.unwrapValue(val),
+        this.chartUtils.unwrapValue,
         figureUpdateEvent
       );
       tableData[column.name] = columnData;