Hi, 

You can go to the table and delete it, here is the link. replace <instance_name> and <incident_sys_id_> in the following link with yours. 

https://<instance_name>.service-now.com/nav_to.do?uri=%2Fsys_journal_field_list.do%3Fsysparm_query%3DnameLIKEincident%255Eelement%253Dcomments%255Eelement_idSTARTSWITH<incident_sys_id>%26sysparm_first_row%3D1%26sysparm_view%3D%26sysparm_choice_query_raw%3D%26sysparm_list_header_search%3Dtrue

Or you can delete by running a background script. 

var gr = new GlideRecord("sys_journal_field");
gr.addQuery("element_id","<incident_sys_id>");
gr.addQuery("element","comments");
gr.deleteMultiple();

Let me know if you have further questions or mark this response as "Helpful/answered" if this helps answer your question.

~Raghu.