"abstract" modifier on enum class is an error

#KT-3454 Fixed

compiler/testData/diagnostics/tests/regressions/Jet183-1.kt

@@ -1,4 +1,4 @@
-abstract enum class ProtocolState {
+enum class ProtocolState {
   WAITING {
     override fun signal() = ProtocolState.TALKING
   }

compiler/testData/diagnostics/tests/regressions/Jet183-1.txt

@@ -2,7 +2,7 @@ package
 
 internal fun box(): kotlin.String
 
-internal abstract enum class ProtocolState : kotlin.Enum<ProtocolState> {
+internal final enum class ProtocolState : kotlin.Enum<ProtocolState> {
     private constructor ProtocolState()
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int

compiler/testData/diagnostics/tests/regressions/Jet183.kt

@@ -1,4 +1,4 @@
-abstract enum class ProtocolState {
+enum class ProtocolState {
   WAITING {
     override fun signal() = ProtocolState.TALKING
   }

compiler/testData/diagnostics/tests/regressions/Jet183.txt

@@ -23,7 +23,7 @@ internal final enum class Foo</*0*/ T> : kotlin.Enum<Foo<T>> {
     public final /*synthesized*/ fun values(): kotlin.Array<Foo<T>>
 }
 
-internal abstract enum class ProtocolState : kotlin.Enum<ProtocolState> {
+internal final enum class ProtocolState : kotlin.Enum<ProtocolState> {
     private constructor ProtocolState()
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int