- Remove some old proto2-based C#-only messages
- Remove the "build" directory which only contained out-of-date files
- Remove the csharp_namespace option from proto2 messages
- Change "Google.ProtocolBuffers" to "Google.Protobuf" in other messages

This is the start of establishing a C# namespace of "Google.ProtocolBuffers.TestProtos.Proto3" for proto3-syntax protos.
We could optionally split the directory structure as well into Proto2 and Proto3 for clarity.