chore: remove intermediate v1.1 pb protocol message type

v1.1.0 was introduced to start the transition to UUID RequestIDs. That
change has since been combined with the switch to DAG-CBOR messaging format
for a v2.0.0 protocol. Thus, this interim v1.1.0 format is no longer needed
and has not been used at all in a released version of go-graphsync.

Fixes: filecoin-project/lightning-planning#14

message/pb/message.proto

@@ -7,7 +7,7 @@ option go_package = ".;graphsync_message_pb";
 message Message {
 
   message Request {
-    bytes id = 1;       // unique id set on the requester side
+    int32 id = 1;       // unique id set on the requester side
     bytes root = 2;     // a CID for the root node in the query
     bytes selector = 3; // ipld selector to retrieve
     map<string, bytes> extensions = 4;    // aux information. useful for other protocols
@@ -17,7 +17,7 @@ message Message {
   }
 
   message Response {
-    bytes id = 1;     // the request id
+    int32 id = 1;     // the request id
     int32 status = 2; // a status code.
     map<string, bytes> extensions = 3; // additional data
   }