Most of the built-in themes have a "listview-stylingless" class that you can apply to a list view to remove the borders, background, and padding.
For reference, there are also classes called "listview-lined" and "listview-striped"
You can remove the border by adding style to your page, inline or using CSS/SASS
Try adding the following to your css file:
.listview-noborder .mx-listview-item {
border: 0;
}
You can then add the 'listview-noborder' class to the listview to remove the border.
[EDIT]
You can add the class in the properties window when selecting the listview element in your page:
You can add the CSS code to the custom.css file, this is located in the following folder: <your project folder>/theme/styles/css/custom