Doesn’t look like you can even cancel it out in XML. Unless a better answer is submitted, you might want to use the ui-silk-shading icon, which might be close to invisible as a mass-action icon.

You’d have to target it with CSS.

.nx-massactions .ui-icon-triangle-1-s {<br>display: none;<br>}

so close, that removes the triangle from the dropdown. I’d like to keep that but remove the icon next to the action label. You gave me everything I needed though, here is what I am doing.
 #UAUpdateTable is the Unique Id of the table the mass actions are on.

#UAUpdateTable .nx-massactions .nx-actionselect-dropdown-item .sk-icon-table-edit {
    display: none !important;
}
#UAUpdateTable .nx-massactions .nx-actionselect-dropdown-item .sk-icon-magic {
    display: none !important;
}