K2 CLI: minor, remove misleading warning

Light tree is used by default now, so it's no longer far from being
production-ready.

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/CommonCompilerArguments.kt

@@ -534,7 +534,7 @@ abstract class CommonCompilerArguments : CommonToolArguments() {
 
     @Argument(
         value = "-Xuse-fir-lt",
-        description = "Compile using LightTree parser with Front-end IR. Warning: this feature is far from being production-ready"
+        description = "Compile using LightTree parser with Front-end IR"
     )
     var useFirLT = true
         set(value) {