[FIR] Get rid of FirSealedClass node

Now list of inheritors is hold in attributes of `FirRegularClass`

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/BuilderConfigurator.kt

@@ -87,11 +87,6 @@ object BuilderConfigurator : AbstractBuilderConfigurator<FirTreeBuilder>(FirTree
             openBuilder()
         }
 
-        builder(sealedClass) {
-            parents += regularClassBuilder
-            defaultNull("companionObject")
-        }
-
         builder(anonymousObject) {
             parents += classBuilder
         }

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/FirTreeBuilder.kt

@@ -46,7 +46,6 @@ object FirTreeBuilder : AbstractFirTreeBuilder() {
     val classLikeDeclaration = element("ClassLikeDeclaration", Declaration, annotatedDeclaration, statement, symbolOwner)
     val klass = element("Class", Declaration, classLikeDeclaration, statement, typeParameterRefsOwner)
     val regularClass = element("RegularClass", Declaration, memberDeclaration, typeParameterRefsOwner, klass)
-    val sealedClass = element("SealedClass", Declaration, regularClass)
     val typeAlias = element("TypeAlias", Declaration, classLikeDeclaration, memberDeclaration, typeParametersOwner)
 
     val function = element("Function", Declaration, callableDeclaration, targetElement, typeParameterRefsOwner, statement)

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/ImplementationConfigurator.kt

@@ -37,8 +37,6 @@ object ImplementationConfigurator : AbstractFirTreeImplementationConfigurator()
 
         impl(regularClass, "FirClassImpl", regularClassConfig)
 
-        impl(sealedClass, config = regularClassConfig)
-
         impl(anonymousObject)
 
         impl(typeAlias)

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/NodeConfigurator.kt

@@ -253,10 +253,6 @@ object NodeConfigurator : AbstractFieldConfigurator<FirTreeBuilder>(FirTreeBuild
             +controlFlowGraphReferenceField
         }
 
-        sealedClass.configure {
-            +fieldList("inheritors", classIdType, withReplace = true)
-        }
-
         typeAlias.configure {
             +typeParameters
             parentArg(classLikeDeclaration, "F", typeAlias)