Remove useless includes of bdd_expr.cpp

These includes are not used

Author: romainbrenguier

src/solvers/prop/bdd_expr.cpp

@@ -11,14 +11,11 @@ Author: Michael Tautschnig, [email protected]
 
 #include "bdd_expr.h"
 
-#include <util/arith_tools.h>
 #include <util/expr_util.h>
 #include <util/format_expr.h>
 #include <util/invariant.h>
 #include <util/std_expr.h>
 
-#include <sstream>
-
 bddt bdd_exprt::from_expr_rec(const exprt &expr)
 {
   PRECONDITION(expr.type().id() == ID_bool);