[FIR] Handle 'EnhancedNullability' more properly

This commit includes three changes:
1. 'EnhancedNullability' is no more set for declaration types
2. It is no more used for conversion types in translator
3. Translator inserts implicit not-null cast only when enhanced type is cast to not-null type.

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/types/irTypes.kt

@@ -50,6 +50,18 @@ fun IrType.addAnnotations(newAnnotations: List<IrConstructorCall>): IrType =
             this
     }
 
+fun IrType.removeAnnotations(predicate: (IrConstructorCall) -> Boolean): IrType =
+    when (this) {
+        is IrSimpleType ->
+            toBuilder().apply {
+                annotations = annotations.filterNot(predicate)
+            }.buildSimpleType()
+        is IrDynamicType ->
+            IrDynamicTypeImpl(null, annotations.filterNot(predicate), Variance.INVARIANT)
+        else ->
+            this
+    }
+
 val IrType.classifierOrFail: IrClassifierSymbol
     get() = cast<IrSimpleType>().classifier