Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/bench/perf_monitoring/gemvt.cpp

@@ -0,0 +1,8 @@
+#include "gemv_common.h"
+
+EIGEN_DONT_INLINE
+void gemv(const Mat &A, Vec &B, const Vec &C) {
+  B.noalias() += A.transpose() * C;
+}
+
+int main(int argc, char **argv) { return main_gemv(argc, argv, gemv); }