Reverts third_party/protobuf: Update to HEAD (f52e188fe4)

third_party/protobuf/csharp/src/Google.Protobuf.Test/ByteStringTest.cs

Index: third_party/protobuf/csharp/src/Google.Protobuf.Test/ByteStringTest.cs
diff --git a/third_party/protobuf/csharp/src/Google.Protobuf.Test/ByteStringTest.cs b/third_party/protobuf/csharp/src/Google.Protobuf.Test/ByteStringTest.cs
index 3ba8df6471f2f56925693dac0881b64f48c78b7e..8935b7829d4a95a703362b5eb317223ef66ae0b8 100644
--- a/third_party/protobuf/csharp/src/Google.Protobuf.Test/ByteStringTest.cs
+++ b/third_party/protobuf/csharp/src/Google.Protobuf.Test/ByteStringTest.cs
@@ -167,18 +167,5 @@ namespace Google.Protobuf
             // Optimization which also fixes issue 61.
             Assert.AreSame(ByteString.Empty, ByteString.FromBase64(""));
         }
-
-        [Test]
-        public void GetHashCode_Regression()
-        {
-            // We used to have an awful hash algorithm where only the last four
-            // bytes were relevant. This is a regression test for
-            // https://github.com/google/protobuf/issues/2511
-
-            ByteString b1 = ByteString.CopyFrom(100, 1, 2, 3, 4);
-            ByteString b2 = ByteString.CopyFrom(200, 1, 2, 3, 4);
-            Assert.AreNotEqual(b1.GetHashCode(), b2.GetHashCode());
-        }
-
     }
 }