remove optional (+)/1 prefix in get_atts/2 and put_atts/2 calls

remove optional (+)/1 prefix in get_atts/2 and put_atts/2 calls

The (+)/1 prefix in get_atts/2 at line 4219 by itself already causes a
greater than 15% slowdown for the benchmark shown in #1730:

    ?- N #= 2^14,
       time(((between(1, N, _),
              X #\= Y,
              false)
            ; true)).

The performance impact is not a good reason to remove the optional
(+)/1 prefix! Performance issues should be addressed at the root, in
this case get_atts/2 (#1962). We should never manually work around
performance issues in built-in predicates.

In contrast, readability is a good argument, and I find the calls
slightly easier to read without the optional (+)/1 prefix.

The prefix is now consistently omitted when possible.