[serializer] remove some dead code.

src/snapshot/snapshot-source-sink.h

Index: src/snapshot/snapshot-source-sink.h
diff --git a/src/snapshot/snapshot-source-sink.h b/src/snapshot/snapshot-source-sink.h
index af617ccee1f4baf9ac53c52e3a584534653d53b9..360ec76bb61986deabbfd20d156dd120ce34fd50 100644
--- a/src/snapshot/snapshot-source-sink.h
+++ b/src/snapshot/snapshot-source-sink.h
@@ -57,9 +57,8 @@ class SnapshotByteSource final {
     return answer;
   }
 
-  bool GetBlob(const byte** data, int* number_of_bytes);
-
-  bool AtEOF();
+  // Returns length.
+  int GetBlob(const byte** data);
 
   int position() { return position_; }