Prunes the grouped header from the header container
@private
function() {
        var me = this,
            header = me.getGroupedHeader();

        if (me.hideGroupedHeader && header) {
            Ext.suspendLayouts();
            if (me.prunedHeader && me.prunedHeader !== header) {
                me.prunedHeader.show();
            }
            me.prunedHeader = header;
            header.hide();
            Ext.resumeLayouts(true);
        }
    }