Your original code couldn’t have worked, because you had the class names the wrong way round.
.jobs-title-display .showing_jobs {visibility:hidden !important;} targets something with a class of .showing_jobs which is a descendant of .jobs-title-display, but your HTML code was the other way round. (If you need a refresher, this article should help: http://www.sitepoint.com/web-foundations/css-selectors/).