I disabled the KDE wallet subsystem in system settings and even uninstalled kwalletmanager but when I open Opera still kwallet services pop-up appears asking for password. It is definitely a bug.
This application is so buggy that it should be optional, definitely not a part of Plasma desktop. Unfortunately I can't uninstall all files that has kwallet in their names because they have a major part of KDE as a dependencies.

Edit:
I entered an empty password and it works. kwallet services don't harass me anymore.