remove three unused includes from util/mp_arith.h

util/mp_arith.h is widely used, so this improves compile times.

Author: Daniel Kroening

src/util/arith_tools.h

@@ -10,12 +10,13 @@ Author: Daniel Kroening, kroening@kroening.com
 #ifndef CPROVER_UTIL_ARITH_TOOLS_H
 #define CPROVER_UTIL_ARITH_TOOLS_H
 
+#include "deprecate.h"
 #include "invariant.h"
 #include "mp_arith.h"
 #include "optional.h"
 #include "std_expr.h"
 
-#include "deprecate.h"
+#include <limits>
 
 class typet;
 

src/util/mp_arith.h

@@ -11,11 +11,8 @@ Author: Daniel Kroening, kroening@kroening.com
 #define CPROVER_UTIL_MP_ARITH_H
 
 #include <string>
-#include <iosfwd>
-#include <limits>
 
 #include "big-int/bigint.hh"
-#include "optional.h"
 #include "deprecate.h"
 
 // NOLINTNEXTLINE(readability/identifiers)