[FIR] Add exhaustive checker for when expressions

Also remove generating default else branch introduced in 38fa0122 and 54eb4cf9

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

@@ -425,6 +425,7 @@ object ImplementationConfigurator : AbstractFirTreeImplementationConfigurator()
 
         impl(whenExpression) {
             default("calleeReference", "FirStubReference()")
+            defaultFalse("isExhaustive")
             useTypes(stubReferenceType)
         }
 

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

@@ -500,6 +500,7 @@ object NodeConfigurator : AbstractFieldConfigurator() {
             +field("subject", expression, nullable = true).withTransform()
             +field("subjectVariable", variable.withArgs("F" to "*"), nullable = true)
             +fieldList("branches", whenBranch).withTransform()
+            +booleanField("isExhaustive", withReplace = true)
             needTransformOtherChildren()
         }