Drop iostream from solver.cc

Change-Id: I512a247c183113ef5dcf09e13f19c17c83293f21

internal/ceres/solver.cc

diff --git a/internal/ceres/solver.cc b/internal/ceres/solver.cc
index e5895f2..3a57084 100644
--- a/internal/ceres/solver.cc
+++ b/internal/ceres/solver.cc
@@ -32,7 +32,6 @@
 #include "ceres/internal/port.h"
 #include "ceres/solver.h"
 
-#include <iostream>  // NOLINT
 #include <sstream>   // NOLINT
 #include <vector>