Turn off by default / remove JS IR transformations

[mvicsokolova]

It should be investigated if JS IR transformations make any performance profit on the JS backend.
If not, it's worth considering to temporarily remove support for the JS IR backend in the compiler plugin and keep kotlinx-atomicfu as an implementation dependency for the JS backend.

[lppedd]

Other than performance, we should also consider bundle size.
Kotlin's stdlib already adds quite a bit to the total size, so if libraries can shave off some KBs, it's always nice.