Remove obsolete unnecessary suppression (#2443)

diff --git a/core/commonMain/src/kotlinx/serialization/internal/PluginGeneratedSerialDescriptor.kt b/core/commonMain/src/kotlinx/serialization/internal/PluginGeneratedSerialDescriptor.kt
index 7b6efe7..a954bda 100644
--- a/core/commonMain/src/kotlinx/serialization/internal/PluginGeneratedSerialDescriptor.kt
+++ b/core/commonMain/src/kotlinx/serialization/internal/PluginGeneratedSerialDescriptor.kt
@@ -1,7 +1,7 @@
 /*
  * Copyright 2017-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
  */
-@file:Suppress("OPTIONAL_DECLARATION_USAGE_IN_NON_COMMON_SOURCE", "UNUSED")
+@file:Suppress("UNUSED")
 
 package kotlinx.serialization.internal