Removed check for existence of first submap of first trajectory in pose graph optimization (#1099)

Removed check for existence of first submap of first trajectory.

This check is equivalent to the original implementation which got changed in [#584](https://github.com/googlecartographer/cartographer/pull/584/files)

cartographer/mapping/internal/optimization/optimization_problem_3d.cc

@@ -278,7 +278,6 @@ void OptimizationProblem3D::Solve(
 
   // Set the starting point.
   CHECK(!submap_data_.empty());
-  CHECK(submap_data_.Contains(SubmapId{0, 0}));
   MapById<SubmapId, CeresPose> C_submaps;
   MapById<NodeId, CeresPose> C_nodes;
   std::map<std::string, CeresPose> C_landmarks;