Deleting old rows when a new one is added and there are more than 100 rows
Your trigger is not specifying which row to delete and is only limiting it to deleting a single record. You are going to want to update the trigger to specify which record to delete and I would guess that you want to delete the oldest one.

https://dev.mysql.com/doc/refman/5.6/en/delete.html


  The LIMIT clause places a limit on the number of rows that can be deleted