Reverts third_party/protobuf: Update to HEAD (f52e188fe4)

third_party/protobuf/csharp/README.md

 This directory contains the C# Protocol Buffers runtime library.
 
+Status: Beta - ready for external testing
+=========================================
+
 Usage
 =====
 
 The easiest way how to use C# protobufs is via the `Google.Protobuf`
 NuGet package. Just add the NuGet package to your VS project.
 
-You will also want to install the `Google.Protobuf.Tools` NuGet package, which
-contains precompiled version of `protoc.exe` and a copy of well known `.proto`
+Besides C# runtime library, the NuGet package also contains 
+precompiled version of `protoc.exe` and a copy of well known `.proto`
 files under the package's `tools` directory.
 
 To generate C# files from your `.proto` files, invoke `protoc` with the 
 `--csharp_out` option.
 
 Supported platforms
 ===================
 
 The runtime library is built as a portable class library, supporting:
 
@@ skipping 37 matching lines @@
 - The old code was based on immutable message types and builders for
 them
 - The old code did not support maps or `oneof`
 - The old code had its own JSON representation, whereas the new code
 uses the standard protobuf JSON representation
 - The old code had no notion of the "well-known types" which have
 special support in the new code
 - The old project supported some older platforms (such as older
 versions of Silverlight) which are not currently supported in the
 new project