Removing duplicate include directive.

Change-Id: I729ae6501497746d1bb615cb893ad592e16ddf3f

internal/ceres/gradient_checker.cc

diff --git a/internal/ceres/gradient_checker.cc b/internal/ceres/gradient_checker.cc
index 219027c..c16c141 100644
--- a/internal/ceres/gradient_checker.cc
+++ b/internal/ceres/gradient_checker.cc
@@ -31,7 +31,6 @@
 //          dgossow@google.com (David Gossow)
 
 #include "ceres/gradient_checker.h"
-#include "ceres/is_close.h"
 
 #include <algorithm>
 #include <cmath>