This can be done using CSS - either within a css file in your app's static folder that is referenced by your dashboard xml, or within dashboard.css to apply across all dashboards.
/* Hide Real-time Options (Splunk 6.4.1) */
.shared-timerangepicker-dialog div.accordion-group:nth-child(3),
.shared-timerangepicker-dialog-presets .presets-group:first-child,
.shared-timerangepicker-dialog-presets .presets-divider-wrap:first-child
{
display: none !important;
}
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