Revert MSVC compile fix as it is already addressed in CMakeLists.txt

Change-Id: I2ea89c6de7593d77cdf31636686beb05ae6833e9

examples/ellipse_approximation.cc

diff --git a/examples/ellipse_approximation.cc b/examples/ellipse_approximation.cc
index 7fd8906..7b2bf12 100644
--- a/examples/ellipse_approximation.cc
+++ b/examples/ellipse_approximation.cc
@@ -35,7 +35,6 @@
 // dynamic_sparsity, and how it can benefit problems which are numerically
 // dense but dynamically sparse.
 
-#define _USE_MATH_DEFINES
 #include <cmath>
 #include <vector>
 #include "ceres/ceres.h"