Remove redundant check in WhenByPlatformEnumChecker

If a type is annotated as not null its upper bound is not nullable
in most cases besides override conflicts, but in the latter case
we should behave consistently with flexibility of this type

compiler/frontend.java/src/org/jetbrains/kotlin/resolve/jvm/checkers/WhenByPlatformEnumChecker.kt

@@ -17,7 +17,6 @@
 package org.jetbrains.kotlin.resolve.jvm.checkers
 
 import org.jetbrains.kotlin.cfg.WhenChecker
-import org.jetbrains.kotlin.load.java.lazy.types.isMarkedNotNull
 import org.jetbrains.kotlin.psi.KtExpression
 import org.jetbrains.kotlin.psi.KtWhenExpression
 import org.jetbrains.kotlin.resolve.BindingContext
@@ -37,7 +36,7 @@ class WhenByPlatformEnumChecker : AdditionalTypeChecker {
             // Check for conditionally-exhaustive when on platform enums, see KT-6399
             val subjectExpression = expression.subjectExpression ?: return
             val type = c.trace.getType(subjectExpression) ?: return
-            if (type.isFlexible() && TypeUtils.isNullableType(type.asFlexibleType().upperBound) && !type.annotations.isMarkedNotNull()) {
+            if (type.isFlexible() && TypeUtils.isNullableType(type.asFlexibleType().upperBound)) {
                 val dataFlowValue = DataFlowValueFactory.createDataFlowValue(subjectExpression, type, c)
                 val dataFlowInfo = c.trace[BindingContext.EXPRESSION_TYPE_INFO, subjectExpression]?.dataFlowInfo
                 if (dataFlowInfo != null && !dataFlowInfo.getStableNullability(dataFlowValue).canBeNull()) {
@@ -55,4 +54,4 @@ class WhenByPlatformEnumChecker : AdditionalTypeChecker {
         }
     }
 
-}
+}