[bugfix] visualize flow error: 'Metric x is not valid' (#3181)

The metric name in the frontend doesn't match the one generated on the
backend. It turns out the explore view will default to the first
metric so specifying one isn't needed.

superset/assets/javascripts/SqlLab/components/VisualizeModal.jsx

@@ -146,18 +146,13 @@ class VisualizeModal extends React.PureComponent {
     this.props.actions.createDatasource(this.buildVizOptions(), this)
       .done(() => {
         const columns = Object.keys(this.state.columns).map(k => this.state.columns[k]);
-        const mainMetric = columns.filter(d => d.agg)[0];
         const mainGroupBy = columns.filter(d => d.is_dim)[0];
         const formData = {
           datasource: this.props.datasource,
           viz_type: this.state.chartType.value,
           since: '100 years ago',
           limit: '0',
         };
-        if (mainMetric) {
-          formData.metrics = [mainMetric.name];
-          formData.metric = mainMetric.name;
-        }
         if (mainGroupBy) {
           formData.groupby = [mainGroupBy.name];
         }