[FIR] Get rid of FirSealedClass node

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

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/collectors/AbstractDiagnosticCollector.kt

@@ -97,10 +97,6 @@ abstract class AbstractDiagnosticCollector(
             visitClassAndChildren(regularClass, regularClass.defaultType())
         }
 
-        override fun visitSealedClass(sealedClass: FirSealedClass) {
-            visitClassAndChildren(sealedClass, sealedClass.defaultType())
-        }
-
         override fun visitAnonymousObject(anonymousObject: FirAnonymousObject) {
             visitClassAndChildren(anonymousObject, anonymousObject.defaultType())
         }

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/collectors/components/DeclarationCheckersDiagnosticComponent.kt

@@ -23,10 +23,6 @@ class DeclarationCheckersDiagnosticComponent(collector: AbstractDiagnosticCollec
         runCheck { checkers.memberDeclarationCheckers.check(regularClass, data, it) }
     }
 
-    override fun visitSealedClass(sealedClass: FirSealedClass, data: CheckerContext) {
-        runCheck { checkers.memberDeclarationCheckers.check(sealedClass, data, it) }
-    }
-
     override fun visitSimpleFunction(simpleFunction: FirSimpleFunction, data: CheckerContext) {
         runCheck { checkers.memberDeclarationCheckers.check(simpleFunction, data, it) }
     }