Remove Joao from CODEOWNERS

Motivation

I honestly currently don't have time to support this project and I don't have much familiarity with it either. At the time, we added myself as a codeowner here to try to support Timo, which was the sole maintainer of the project. I was never an active maintainer here, and I was given permission more to support in an emergency basis when Timo would not be available.

Having myself as CODEOWNER makes me get pinged for reviews by the bot. E.g.: !1912 (comment 1880422109)

When this happens I've been 100% of the time delegating to @PatrickRice or @timofurrer, which do an amazing job of picking up these MRs from me even before I even ping them! Probably they are subscribing to new MR pushes/labels, which is awesome! 🏆

Now that we have also Patrick as developer/maintainer, and Nico and Viktor are anyway also owners, I don't think it makes much sense for me to hold this emergency-only CODEOWNER seat, until I'd have enough time to meaningfully contribute to this project.