Un-deprecate to_integer(constant_exprt)

The recommended substitute uses this function, removing it is currently
impossible.

Author: tautschnig

src/util/arith_tools.h

@@ -26,8 +26,6 @@ DEPRECATED("Use the constant_exprt version instead")
 bool to_integer(const exprt &expr, mp_integer &int_value);
 
 // returns 'true' on error
-/// \deprecated: use numeric_cast<mp_integer> instead
-DEPRECATED("Use numeric_cast<mp_integer> instead")
 bool to_integer(const constant_exprt &expr, mp_integer &int_value);
 
 // returns 'true' on error