Fix build warnings around unused variables

csharp/src/Google.Protobuf.Test/FieldCodecTest.cs

@@ -39,6 +39,7 @@ namespace Google.Protobuf
 {
     public class FieldCodecTest
     {
+#pragma warning disable 0414 // Used by tests via reflection - do not remove!
         private static readonly List<ICodecTestData> Codecs = new List<ICodecTestData>
         {
             new FieldCodecTestData<bool>(FieldCodec.ForBool(100), true, "Bool"),
@@ -61,6 +62,7 @@ namespace Google.Protobuf
             new FieldCodecTestData<ForeignMessage>(
                 FieldCodec.ForMessage(100, ForeignMessage.Parser), new ForeignMessage { C = 10 }, "Message"),
         };
+#pragma warning restore 0414
 
         [Test, TestCaseSource("Codecs")]
         public void RoundTripWithTag(ICodecTestData codec)

csharp/src/Google.Protobuf.Test/IssuesTest.cs

@@ -49,8 +49,7 @@ namespace Google.Protobuf
             ItemField message = new ItemField { Item = 3 };
             FieldDescriptor field = ItemField.Descriptor.FindFieldByName("item");
             Assert.NotNull(field);
-            // TODO(jonskeet): Reflection...
-            // Assert.AreEqual(3, (int)message[field]);
+            Assert.AreEqual(3, (int)field.Accessor.GetValue(message));
         }
 
         [Test]

csharp/src/Google.Protobuf/JsonFormatter.cs

@@ -739,7 +739,6 @@ namespace Google.Protobuf
         private const string Hex = "0123456789abcdef";
         private static void HexEncodeUtf16CodeUnit(StringBuilder builder, char c)
         {
-            uint utf16 = c;
             builder.Append("\\u");
             builder.Append(Hex[(c >> 12) & 0xf]);
             builder.Append(Hex[(c >> 8) & 0xf]);