Revert kotlinx-serialization-compiler-plugin OptIn dependency

plugins/kotlin-serialization/kotlin-serialization-compiler/build.gradle.kts

@@ -47,11 +47,3 @@ projectTest(parallel = true) {
 
 apply(from = "$rootDir/gradle/kotlinPluginPublication.gradle.kts")
 
-
-tasks {
-    val compileKotlin by existing(org.jetbrains.kotlin.gradle.tasks.KotlinCompile::class) {
-        kotlinOptions {
-            freeCompilerArgs += "-Xopt-in=org.jetbrains.kotlin.ir.ObsoleteDescriptorBasedAPI"
-        }
-    }
-}