remove unused this capture

Author: scotthart

google/cloud/bigtable/internal/data_connection_impl.cc

@@ -879,7 +879,7 @@ future<StatusOr<bigtable::PreparedQuery>> DataConnectionImpl::AsyncPrepareQuery(
   return google::cloud::internal::AsyncRetryLoop(
              std::move(retry), std::move(backoff), Idempotency::kIdempotent,
              background_->cq(),
-             [this, stub, instance_name, operation_context](
+             [stub, instance_name, operation_context](
                  CompletionQueue& cq,
                  std::shared_ptr<grpc::ClientContext> context,
                  google::cloud::internal::ImmutableOptions options,