Deleting data from workflow history list using JSON 

copy past the below code in script editor webpart.

<script src="https://sharepoint_yoursite/subsitename/SiteAssets/Java%20Script%20Files/jquery-1.10.2.js"></script>
    <script type="text/javascript">
var clientContext;
var website;
var oList;
var cnt = 0;
var collListItem;
// Make sure the SharePoint script file 'sp.js' is loaded before your code runs.
SP.SOD.executeFunc('sp.js', 'SP.ClientContext', sharePointReady);

// Create an instance of the current context.
function sharePointReady() {
    clientContext = SP.ClientContext.get_current();
    website = clientContext.get_web();
    oList = website.get_lists().getByTitle('Workflow History');

    var camlQuery = new SP.CamlQuery();
    camlQuery.set_viewXml('<View><RowLimit>1000</RowLimit></View>');
    this.collListItem = oList.getItems(camlQuery);

    clientContext.load(website);
    clientContext.load(collListItem, 'Include(Id)');
    clientContext.executeQueryAsync(Function.createDelegate(this, this.onQuerySucceeded), Function.createDelegate(this, this.onQueryFailed));
}

function onQuerySucceeded(sender, args) {

    var listItemInfo = '';
    var listItemEnumerator = collListItem.getEnumerator();

    while (listItemEnumerator.moveNext()) {
        var oListItem = listItemEnumerator.get_current();
        var ID = oListItem.get_id();
        var oListItemDel = oList.getItemById(ID);
        oListItemDel.deleteObject();
        clientContext.executeQueryAsync(Function.createDelegate(this, this.onDeleteSucceeded), Function.createDelegate(this, this.onDeleteFailed));
        console.log(ID + " : Deleted");
    }
}

function onQueryFailed(sender, args) {
    console.log('Request failed. ' + args.get_message() + '\n' + args.get_stackTrace());
}


function onDeleteFailed(sender, args) {
    console.log('Delete failed. ' + args.get_message() + '\n' + args.get_stackTrace());
}

function onDeleteSucceeded(sender, args) {
    cnt = cnt + 1;
    console.log('Delete success : ' + cnt);
}
</script>