@ddpweb – apparently the only way to do so officially is to pay for one of the many extra bits that allows you to control the application process.
I have found no easy way of making it optional otherwise, as no matter what I change if the field is there the javascript validation insists that it is filled, even when the class is changed to not-required
which seems a little odd.
However if (like me) you want to remove the resume upload from all jobs then, because the validation code is rather poor, it is very easy to simply comment out line 431 in the file simple-job-board/templates/v1/single-jobpost/job-application.php
(or which ever template you are using).
There were a lot of things I ended up changing to make the plugin work for my client, so in the end I made my own version. However the documentation seems to suggest it is possible to override these templates and so if this is the only change you are making it may be worth pursuing that direction.
Hope that helps you out.