[K/JS Gradle] Remove forceLegacyBackendUsage from KotlinGradlePlugin previously added with the compiler flags refactoring, also fix bootstrap compilation of atomicfu plugin

libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/tasks/Kotlin2JsCompile.kt

@@ -158,10 +158,6 @@ abstract class Kotlin2JsCompile @Inject constructor(
                 args.fragmentRefines = multiplatformStructure.fragmentRefinesCompilerArgs
             }
 
-            if (!isIrBackendEnabled()) {
-                args.forceDeprecatedLegacyCompilerUsage = true
-            }
-
             // Overriding freeArgs from compilerOptions with enhanced one + additional one set on execution phase
             // containing additional arguments based on the js compilation configuration
             args.freeArgs = executionTimeFreeCompilerArgs ?: enhancedFreeCompilerArgs.get()