tweak improve code readability in cancel_cmd

tweak improve code readability in cancel_cmd

- it is useless to test for nb != -1 because nb is always >=0
- maybe the idea behind the comment was to test for ret and not nb ???
- &= is a bitwise operator, not for boolean
- notice that the two lines below don't work the same because
    ret = cancel_job(ua, jcr, 60, cancel) && ret;
    ret = ret && cancel_job(ua, jcr, 60, cancel);
  I chose the first one, but maybe because of the comment that I have
  removed, we should use the first one