Remove redundant @OptIn targeting new case conversion api

libraries/tools/kotlin-stdlib-gen/src/generators/unicode/mappings/oneToMany/writers/OneToManyTitlecaseMappingsWriter.kt

@@ -23,7 +23,6 @@ internal class OneToManyTitlecaseMappingsWriter : OneToManyMappingsWriter {
     }
 
     private fun titlecaseImpl(apostropheN: String): String = """
-        @OptIn(ExperimentalStdlibApi::class)
         internal fun Char.titlecaseImpl(): String {
             val uppercase = uppercase()
             if (uppercase.length > 1) {

libraries/tools/kotlin-stdlib-gen/src/generators/unicode/mappings/oneToOne/writers/TitlecaseMappingsWriter.kt

@@ -22,7 +22,6 @@ internal class TitlecaseMappingsWriter : MappingsWriter {
 
         writer.append(
             """
-            @OptIn(ExperimentalStdlibApi::class)
             internal fun Char.titlecaseCharImpl(): Char {
                 val code = this.code
                 // Letters repeating <Lu, Lt, Ll> sequence and code of the Lt is a multiple of 3, e.g. <DŽ, Dž, dž>

libraries/tools/kotlin-stdlib-gen/src/generators/unicode/mappings/string/StringCasingTestGenerator.kt

@@ -67,7 +67,6 @@ internal class StringCasingTestGenerator(private val outputDir: File) {
         }
     }
 
-    @OptIn(ExperimentalStdlibApi::class)
     private fun test(rangesArrayName: String, functionName: String): String = """
         class ${functionName.replaceFirstChar { it.uppercase() }}Test {
             @Test