Remove an unused variable from gradient_checker.cc

Change-Id: Ib2bfb3d1efacf197e14fa04ac18370d35060b322

internal/ceres/gradient_checker.cc

diff --git a/internal/ceres/gradient_checker.cc b/internal/ceres/gradient_checker.cc
index 6ae7d55..c0dcd47 100644
--- a/internal/ceres/gradient_checker.cc
+++ b/internal/ceres/gradient_checker.cc
@@ -142,7 +142,6 @@
 
   const vector<int32_t>& parameter_block_sizes =
       function->parameter_block_sizes();
-  const int num_parameter_blocks = parameter_block_sizes.size();
   for (int32_t parameter_block_size : parameter_block_sizes) {
     finite_diff_cost_function->AddParameterBlock(parameter_block_size);
   }