Remove unused diagnosticEngine property (#1033)

Author: d-ronnqvist

Sources/SwiftDocC/Model/DocumentationNode.swift

@@ -430,10 +430,9 @@ public struct DocumentationNode {
         let symbolAllowedValues = symbol![mixin: SymbolGraph.Symbol.AllowedValues.self]
 
         if let possibleValues = markupModel.discussionTags?.possiblePropertyListValues, !possibleValues.isEmpty {
-            let validator = PropertyListPossibleValuesSection.Validator(diagnosticEngine: engine)
             guard let symbolAllowedValues else {
-                possibleValues.forEach { 
-                    engine.emit(validator.makeExtraPossibleValueProblem($0, knownPossibleValues: [], symbolName: self.name.plainText))
+                for value in possibleValues {
+                    engine.emit(PropertyListPossibleValuesSection.Validator.makeExtraPossibleValueProblem(value, knownPossibleValues: [], symbolName: self.name.plainText))
                 }
                 return
             }
@@ -456,7 +455,7 @@ public struct DocumentationNode {
 
             for unknownValue in unknownPossibleValues {
                 engine.emit(
-                    validator.makeExtraPossibleValueProblem(unknownValue, knownPossibleValues: knownPossibleValueNames, symbolName: self.name.plainText)
+                    PropertyListPossibleValuesSection.Validator.makeExtraPossibleValueProblem(unknownValue, knownPossibleValues: knownPossibleValueNames, symbolName: self.name.plainText)
                 )
             }
 

Sources/SwiftDocC/Model/Section/Sections/PropertyListPossibleValuesSection.swift

@@ -43,10 +43,7 @@ public struct PropertyListPossibleValuesSection {
     /// The list of possible values.
     public let possibleValues: [PossibleValue]
 
-    struct Validator {
-        /// The engine that collects problems encountered while validating the possible values documentation.
-        var diagnosticEngine: DiagnosticEngine
-        
+    enum Validator {
         /// Creates a new problem about documentation for a possible value that's not known to that symbol.
         ///
         /// ## Example
@@ -63,7 +60,7 @@ public struct PropertyListPossibleValuesSection {
         ///   - unknownPossibleValue: The authored documentation for the unknown possible value name.
         ///   - knownPossibleValues: All known possible value names for that symbol.
         /// - Returns: A new problem that suggests that the developer removes the documentation for the unknown possible value.
-        func makeExtraPossibleValueProblem(_ unknownPossibleValue: PossibleValue, knownPossibleValues: Set<String>, symbolName: String) -> Problem {
+        static func makeExtraPossibleValueProblem(_ unknownPossibleValue: PossibleValue, knownPossibleValues: Set<String>, symbolName: String) -> Problem {
 
             let source = unknownPossibleValue.range?.source
             let summary = """