Example shows that fetch is logged under topic info. If you want to hide such messages, then you simply have to disable info topic. There is no other way.
Lets say, if we use as an example L2TP logs, then part of them is logged under info,l2tp topic. Then you can under logging rules change topic from info to info,!l2tp and then all info logs except l2tp ones will be saved.
It would be best when MikroTik would assign some more specific topic to this (and some other) log message.
E.g. when it would not be logged as info but as info,fetch it would be easy to block this log by adding !fetch on the info topic log, and still log other info.
Maybe do a scan of all logging and setup some more topics like this or even better add a unique topic (message code) to each and every message in the system.
The proper solution is more granular logging options, not disabling pretty much ALL logging (since info contains tons of other information) just because fetch is noisy every time you run it. It makes scripting scheduling with fetch a PITA.