Upgrade rust/crates/protobuf-codegen to 2.16.2

Test: make
Change-Id: I14166e5bd65e98c7c7a53cff992df72164346bc5
diff --git a/src/extensions.rs b/src/extensions.rs
index a3f8901..637e18f 100644
--- a/src/extensions.rs
+++ b/src/extensions.rs
@@ -88,9 +88,8 @@
     }
 
     w.write_line("");
+    w.write_line("/// Extension fields");
     w.pub_mod("exts", |w| {
-        w.write_line("use protobuf::Message as Message_imported_for_functions;");
-
         for field in file.get_extension() {
             if field.get_field_type() == FieldDescriptorProto_Type::TYPE_GROUP {
                 continue;