DELETE inside INSERT Trigger
You cannot alter the same table in trigger.

Use rotate (update the oldest record) instead of add and delete. Insert 100 records into your table (if there is no enought records in it) with their timestamp in far past and use

UPDATE logs
SET field1=value1, field2=value2 -- , ...
ORDER BY record_timestamp ASC
LIMIT 1;


The records count will be always unchanged. To increase it simply add the amount of records you need with timestamp in far past, to decrease - delete the amount of oldest records you do not need.

And you do not need in trigger at all...