[kxSerialization] Added diagnostic on useless Serializer annotation

Resolves Kotlin/kotlinx.serialization#2182

Merge-request: KT-MR-8718
Merged-by: Sergey Shanshin <Sergey.Shanshin@jetbrains.com>

plugins/kotlinx-serialization/testData/firMembers/serializerViaCompanion.kt

@@ -67,8 +67,8 @@ data class PartiallyWithoutType(val i: Int) {
 
 @Serializable(FullyOverridden.Companion::class)
 data class FullyOverridden(val i: Int) {
-    @Serializer(forClass = FullyOverridden::class)
     companion object : KSerializer<FullyOverridden> {
+
         override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("FullyOverridden", PrimitiveKind.STRING)
 
         override fun serialize(encoder: Encoder, value: FullyOverridden) {