[FIR/IR generator] Move isSealed property to AbstractElement

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

@@ -56,8 +56,6 @@ class Element(name: String, override val propertyName: String, kind: Kind) : Abs
         }
     var _needTransformOtherChildren: Boolean = false
 
-    override var isSealed: Boolean = false
-
     override val hasAcceptMethod: Boolean
         get() = true
 

generators/tree-generator-common/src/org/jetbrains/kotlin/generators/tree/AbstractElement.kt

@@ -35,8 +35,7 @@ abstract class AbstractElement<Element, Field>(
     val isRootElement: Boolean
         get() = elementParents.isEmpty()
 
-    open val isSealed: Boolean
-        get() = false
+    var isSealed: Boolean = false
 
     /**
      * The value of this property will be used to name a `visit*` method for this element in visitor classes.