Remove JS Legacy transformer

[mvicsokolova]

AtomicfuJSTransformer that transformed the final *.js files for JS legacy compiler backend can be removed from the library, as Kotlin compiler already removed JS legacy backend since Kotlin 1.9.0

[mvicsokolova]

Starting from Kotlin 2.1.0-Beta1 with kotlinx-atomicfu <= 0.25.0 keeping transformJs flag enabled is an error ❌:

e: KLIB resolver: The same 'unique_name=kotlinx.atomicfu.examples:mpp-sample' found in more than one library: /build/classes/atomicfu/js/main, /build/classes/kotlin/js/main
Please file an issue to https://kotl.in/issue and meanwhile use CLI flag `-Xklib-duplicated-unique-name-strategy` with one of the following values:
allow-all-with-warning: Use all KLIB dependencies, even when they have same `unique_name` property.
allow-first-with-warning: Use the first KLIB dependency with clashing `unique_name` property. No order guarantees are given though.

This happens, because in kotlinx-atomicfu 0.25.0 this flag is still enabled by default and while taking no effect for Kotlin versions > 1.9.0 (no transformations are performed, because Js Legacy is disabled since Kotlin 1.9.0), the AtomicfuGradlePlugin still created an empty directory: /build/classes/kotlin/js/main.

In the nearest release 0.25.1 transformJs flag will be deprecated with a warning and will take no effect, the JS Legacy transformer will be removed.
In the next release transformJs flag will be removed completely.