Revert "third_party/protobuf: Update to HEAD (83d681ee2c)"

third_party/protobuf/csharp/src/Google.Protobuf/WellKnownTypes/TimeExtensions.cs

 #region Copyright notice and license
 // Protocol Buffers - Google's data interchange format
 // Copyright 2015 Google Inc.  All rights reserved.
 // https://developers.google.com/protocol-buffers/
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
@@ skipping 13 matching lines @@
 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #endregion
 
 using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
 
 namespace Google.Protobuf.WellKnownTypes
 {
     /// <summary>
     /// Extension methods on BCL time-related types, converting to protobuf types.
     /// </summary>
     public static class TimeExtensions
     {
         /// <summary>
         /// Converts the given <see cref="DateTime"/> to a <see cref="Timestamp"/>.
@@ skipping 23 matching lines @@
         /// Converts the given <see cref="TimeSpan"/> to a <see cref="Duration"/>.
         /// </summary>
         /// <param name="timeSpan">The time span to convert.</param>
         /// <returns>The converted duration.</returns>
         public static Duration ToDuration(this TimeSpan timeSpan)
         {
             return Duration.FromTimeSpan(timeSpan);
         }
     }
 }