Remove unused import
Resolves the compile warning:
[WARNING] PROTOC: runtime.proto:21:1: warning: Import blockchain.proto but not used.
@@ -18,7 +18,6 @@ package exonum.runtime;
option java_package = "com.exonum.core.messages";
-import "blockchain.proto";
import "google/protobuf/empty.proto";
// Unique service transaction identifier.
