[FIR generator] Remove unused notNull property

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

@@ -21,7 +21,6 @@ sealed class Field : AbstractField<Field>() {
     open val fromDelegate: Boolean get() = false
 
     open var useNullableForReplace: Boolean = false
-    open var notNull: Boolean = false
 
     var withBindThis = true
 

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

@@ -49,16 +49,6 @@ abstract class AbstractBuilderPrinter<Element, Implementation, BuilderField, Ele
                     field.name,
                     ")",
                 )
-                field.notNull -> println(
-                    originalParameterName,
-                    ".",
-                    field.name,
-                    "?.let { ",
-                    copyBuilderVariableName,
-                    ".",
-                    field.name,
-                    " = it }",
-                )
                 else -> println(copyBuilderVariableName, ".", field.name, " = ", originalParameterName, ".", field.name)
             }
         }
@@ -211,7 +201,7 @@ abstract class AbstractBuilderPrinter<Element, Implementation, BuilderField, Ele
     }
 
     private fun BuilderField.needBackingField(fieldIsUseless: Boolean) =
-        (!nullable || notNull) && origin !is ListField && if (fieldIsUseless) {
+        !nullable && origin !is ListField && if (fieldIsUseless) {
             defaultValueInImplementation == null
         } else {
             defaultValueInBuilder == null

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

@@ -9,7 +9,6 @@ interface AbstractFieldWithDefaultValue<OriginField : AbstractField<OriginField>
     val origin: OriginField
     var withGetter: Boolean
     var customSetter: String?
-    var notNull: Boolean
     var defaultValueInImplementation: String?
     var defaultValueInBuilder: String?
-}
+}