Have you read How do I get rid of top sites?
I don't have Top Sites in my bookmarks bar but I can make a suggestion if the above doesn't help: Go to Bookmarks > Edit Bookmarks. You may be able to find Top Sites there. If so, you should be able to delete it by selecting it and pressing Delete.
OS 10.14.3, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365