sortablegridfield/css/GridFieldSortableRows.css

.cms table.ss-gridfield-table thead tr th.sortablerowsheading {
    border-bottom: 1px solid rgba(0, 0, 0, 0.1);
}

.cms table.ss-gridfield-table thead tr th.sortablerowsheading .gridfield-sortablerows button {
    display: none;
}