我也没有找到，但可以用下面的重写：

Ext.define('overrides.grid.header.Container', {
    override: 'Ext.grid.header.Container',

    getMenuItems: function() {
        var me = this,
            menuItems = [],
            hideableColumns = me.enableColumnHide ? me.getColumnMenu(me) : null;

        if (me.sortable) {
            menuItems = [{
                itemId: 'ascItem',
                text: me.sortAscText,
                iconCls: me.menuSortAscCls,
                handler: me.onSortAscClick,
                scope: me
            }, {
                itemId: 'descItem',
                text: me.sortDescText,
                iconCls: me.menuSortDescCls,
                handler: me.onSortDescClick,
                scope: me
            }, {
                itemId: 'dropSortItem',
                text: me.sortClearText,
                //iconCls: me.menuSortDescCls, // Your icon
                handler: me.onSortClearClick,
                scope: me
            }];
        }

        if (hideableColumns && hideableColumns.length) {
            if (me.sortable) {
                menuItems.push({
                    itemId: 'columnItemSeparator',
                    xtype: 'menuseparator'
                });
            }

            menuItems.push({
                itemId: 'columnItem',
                text: me.columnsText,
                iconCls: me.menuColsIcon,
                menu: hideableColumns,
                hideOnClick: false
            });
        }

        return menuItems;
    },

    onSortClearClick: function() {
        var menu = this.getMenu(),
            activeHeader = menu.activeHeader,
            store = this.up('grid').getStore();
        store.getSorters().each(function(sorter) {
            if(sorter.initialConfig.property == activeHeader.dataIndex) {
                store.getSorters().remove(sorter)
            }       
        }, this);
    }
});