Remove additional date from notification and lock screen
I found out that it's not something that can be changed via settings - it come's like that out of the box when the device is meant to be sold in a different market.
In my case, every time I insert a new SIM card into the phone and restart it I got a notification saying that in order to set new settings for the phone I must do a factory reset. So when putting back my own SIM I also got this message. Once I did that suggested factory reset phone started fresh with a factory configuration that matches my region, making the Persian calendar disappear and also enabling other features that weren't enabled before, such as native call recording
(worked on both Samsung Galaxy S8 and S20)