Remove duplicate test cases.

(NCrunch noticed these.)

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

@@ -321,7 +321,6 @@ namespace Google.Protobuf
         [TestCase("1970-01-01T00:00:00.001Z", 1000000)]
         [TestCase("1970-01-01T00:00:00.010Z", 10000000)]
         [TestCase("1970-01-01T00:00:00.100Z", 100000000)]
-        [TestCase("1970-01-01T00:00:00.100Z", 100000000)]
         [TestCase("1970-01-01T00:00:00.120Z", 120000000)]
         [TestCase("1970-01-01T00:00:00.123Z", 123000000)]
         [TestCase("1970-01-01T00:00:00.123400Z", 123400000)]

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

@@ -249,7 +249,6 @@ namespace Google.Protobuf
         [TestCase("[,", 1)]
         [TestCase("{", 1)]
         [TestCase("{,", 1)]
-        [TestCase("{", 1)]
         [TestCase("{[", 1)]
         [TestCase("{{", 1)]
         [TestCase("{0", 1)]