[xray] Fix compilation on mac (#2199)

Author: pcmoritz

src/ray/object_manager/object_directory.cc

@@ -148,8 +148,8 @@ ray::Status ObjectDirectory::LookupLocations(const ObjectID &object_id,
   JobID job_id = JobID::nil();
   ray::Status status = gcs_client_->object_table().Lookup(
       job_id, object_id,
-      [this, callback](gcs::AsyncGcsClient *client, const ObjectID &object_id,
-                       const std::vector<ObjectTableDataT> &location_history) {
+      [callback](gcs::AsyncGcsClient *client, const ObjectID &object_id,
+                 const std::vector<ObjectTableDataT> &location_history) {
         // Build the set of current locations based on the entries in the log.
         std::unordered_set<ClientID> client_ids;
         std::vector<ClientID> locations_vector =