remove '#include <iostream>' from headers and from unneeded places

It's harmful to have iostream everywhere as it injects functions in the compiled files

src/shell/lp_frontend.cpp

@@ -15,6 +15,7 @@ Author:
 #include "util/rlimit.h"
 #include "util/gparams.h"
 #include "util/mutex.h"
+#include <iostream>
 #include <signal.h>
 #include "smt/params/smt_params_helper.hpp"
 

src/shell/opt_frontend.cpp

@@ -4,6 +4,7 @@ Copyright (c) 2015 Microsoft Corporation
 --*/
 
 #include<fstream>
+#include<iostream>
 #include<signal.h>
 #include<time.h>
 #include "util/gparams.h"