Update conformance test failure list

conformance/failure_list_csharp.txt

@@ -1,6 +1,2 @@
 Recommended.JsonInput.FieldNameWithDoubleUnderscores.JsonOutput
 Required.JsonInput.FieldNameInSnakeCase.JsonOutput
-Required.JsonInput.FieldNameWithMixedCases.JsonOutput
-Required.JsonInput.FieldNameWithMixedCases.ProtobufOutput
-Required.JsonInput.FieldNameWithMixedCases.Validator
-Required.JsonInput.OriginalProtoFieldName.JsonOutput