As discussed in #3470 this warning can cause unwarranted concern in users.

Following the path of React, where this warning was removed facebook/react#22114 (comment) it can likely be similarly removed from Preact.

Basically, Preact already checks if the component is unmounted, hence its ability to provide the warning, and most users that run into this implement anti-patterns to suppress the warning instead of addressing the actual issue being warned about.

The actual issue is less likely to be an indication of a real memory leak, and more likely to be just a single late update.

With all this in mind, it makes sense to also remove the warning here.

What this does:

• Removes the warning for setting state after a component has unmounted
• Removes test that checked for warning

Resolves #3470