Remove MaxNumThreadsAvailable

It is just a wrapper around ThreadPool::MaxNumThreadsAvailable
and has just one callsite.

Change-Id: Ic4b496c86a9760d1024ff34db305ea8db99705d8

internal/ceres/parallel_for.cc

@@ -44,8 +44,6 @@
 
 namespace ceres::internal {
 
-int MaxNumThreadsAvailable() { return ThreadPool::MaxNumThreadsAvailable(); }
-
 void ParallelSetZero(ContextImpl* context,
                      int num_threads,
                      double* values,

internal/ceres/parallel_for.h

@@ -51,9 +51,6 @@ inline decltype(auto) MakeConditionalLock(const int num_threads,
                             : std::unique_lock<std::mutex>{m};
 }
 
-// Returns the maximum supported number of threads
-CERES_NO_EXPORT int MaxNumThreadsAvailable();
-
 // Execute the function for every element in the range [start, end) with at most
 // num_threads. It will execute all the work on the calling thread if
 // num_threads or (end - start) is equal to 1.

internal/ceres/preprocessor.cc

@@ -35,9 +35,9 @@
 #include "ceres/callbacks.h"
 #include "ceres/gradient_checking_cost_function.h"
 #include "ceres/line_search_preprocessor.h"
-#include "ceres/parallel_for.h"
 #include "ceres/problem_impl.h"
 #include "ceres/solver.h"
+#include "ceres/thread_pool.h"
 #include "ceres/trust_region_preprocessor.h"
 
 namespace ceres::internal {
@@ -62,7 +62,7 @@ void ChangeNumThreadsIfNeeded(Solver::Options* options) {
   if (options->num_threads == 1) {
     return;
   }
-  const int num_threads_available = MaxNumThreadsAvailable();
+  const int num_threads_available = ThreadPool::MaxNumThreadsAvailable();
   if (options->num_threads > num_threads_available) {
     LOG(WARNING) << "Specified options.num_threads: " << options->num_threads
                  << " exceeds maximum available from the threading model Ceres "