AtomicFu's bytecode transformer transforms all classes, even if they do not use atomicfu

[ZacSweers]

It appears that AtomicFu's bytecode transformer deserializes and re-serializes @kotlin.Metadata for all classes, even those with no atomicfu references. This exacerbates issues like #578, which will now happen regardless even if the class being processed has nothing to do with AtomicFu.

[fzhinkin]

The plan is to sunset the bytecode transformer and use only the IR transformer (aka the compiler plugin).