This is what I do, add the code to template extra.less. Whatever widget - first get the widget key (click in the widget settings in the widget list). This example is for whats_new_new_posts
CSS:
[data-widget-key="whats_new_new_posts"] .contentRow-minor,
[data-widget-key="whats_new_new_posts"] .contentRow .label
{display:none}