More cleanup, based around searches for "Google.ProtocolBuffers"

- 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- Remove the "build" directory which only contained out-of-date files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

Author: jskeet

benchmarks/google_size.proto

@@ -4,7 +4,6 @@ package benchmarks;
 
 option java_outer_classname = "GoogleSize";
 option optimize_for = CODE_SIZE;
-option csharp_namespace = "Google.ProtocolBuffers.TestProtos";
 
 message SizeMessage1 {
   required string field1 = 1;

benchmarks/google_speed.proto

@@ -4,7 +4,6 @@ package benchmarks;
 
 option java_outer_classname = "GoogleSpeed";
 option optimize_for = SPEED;
-option csharp_namespace = "Google.ProtocolBuffers.TestProtos";
 
 message SpeedMessage1 {
   required string field1 = 1;