Remove QAtomic*

It seems the QAtomic classes have reached the end of their usefulness. The only non-std::atomic-backed implementation is bootstrap (ie. non-atomic), and only moc and automoc remain bootstrapped. So QAtomic is no longer useful as an abstraction of platform-specific atomic operations, esp. seeing as libstdc++, at least, already uses non-locked operations for std::atomic if the application is single-threaded.

At the same time that QAtomic lost its defining meaning, upstream charges ahead and adds wait/notify semantics to every std::atomic in C++20. To gain feature parity, while maintaining compatibility with C++17, we'd need to re-imlement this functionality using futexes. This doesn't seem like a good use of our time.

Therefore, this seems to be a good point time, before somone spends the time to implement QAtomic::wait(), to put QAtomic to rest.

Target should be to remove the classes for Qt 7.

Issues in this epic will define the roadmap until complete removal of the classes.