Remove OpenMP and No threading backends.

Since c++11, we can depend on C++ threads always being available.
With the recent work on the performance of CXX threading, the
additional complexity of maintaining multiple backends for some
minor performance delta is not worth it

https://github.com/ceres-solver/ceres-solver/issues/886

Change-Id: Idee480b22a498daec9c4366da8589aa58eaf36a1

internal/ceres/covariance_impl.cc

@@ -65,15 +65,6 @@ using CovarianceBlocks = std::vector<std::pair<const double*, const double*>>;
 
 CovarianceImpl::CovarianceImpl(const Covariance::Options& options)
     : options_(options), is_computed_(false), is_valid_(false) {
-#ifdef CERES_NO_THREADS
-  if (options_.num_threads > 1) {
-    LOG(WARNING) << "No threading support is compiled into this binary; "
-                 << "only options.num_threads = 1 is supported. Switching "
-                 << "to single threaded mode.";
-    options_.num_threads = 1;
-  }
-#endif
-
   evaluate_options_.num_threads = options_.num_threads;
   evaluate_options_.apply_loss_function = options_.apply_loss_function;
 }