Deleting an Insight you have defined, whether a chart, table, pivot, or other type of insight, is quite easy. Select the Insight from the list of Insights in a library. Then select Delete from the Selected Chart menu. Note that if you do not see the option to delete, it is most likely that this insight is used in a dashboard, and so cannot be currently deleted.
On the confirmation screen, click Yes if you are ready to delete the chart.