Resolve some warnings in operations

Author: alex-kulakov

Extensions/Xtensive.Orm.Operations/Operations/KeySetOperation.cs

@@ -18,8 +18,6 @@ namespace Xtensive.Orm.Operations
   /// </summary>
   public abstract class KeySetOperation : Operation
   {
-    private readonly IReadOnlyList<Key> keys;
-
     /// <inheritdoc/>
     [JsonIgnore]
     public override string Description {
@@ -33,10 +31,7 @@ public override string Description {
     /// Gets the key set.
     /// </summary>
     [JsonInclude]
-    public IReadOnlyList<Key> Keys {
-      get;
-      private set;
-     }
+    public IReadOnlyList<Key> Keys { get; }
 
     /// <inheritdoc/>
     protected override void PrepareSelf(OperationExecutionContext context)

Extensions/Xtensive.Orm.Operations/Serialization/Json/Converters/KeyConverterFactory.cs

@@ -38,9 +38,11 @@ public override void Write(Utf8JsonWriter writer, Key value, JsonSerializerOptio
 
     private readonly Domain domain = domain;
 
+    /// <inheritdoc/>
     public override bool CanConvert(Type typeToConvert) =>
       typeToConvert.IsAssignableTo(typeof(Key));
 
+    /// <inheritdoc/>
     public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options)
     {
       // there might be implementation of per-type converters