This is not very important actually. This query is ran constantly to clean-up the MLM subscription folder and remove un-handled requests. You may cause that error yourself when clicking on test messages...
I'm getting this error a lot now too. Should I worry?
Subject: "[MLM] Error deleting record from RemoveSubscriberFromDB()"
Body Text: ": DELETE LOW_PRIORITY FROM `tbl_mlm_sub` WHERE"
Max Bulk Mailer version: 8.6.9
MLM version: 3.1.9
PHP version: 7.3.11 (Safe mode OFF)
Mac OSX version: 10.14.6