Remove the Unable to find 'webwork.multipart.saveDir' property setting message. It confuses people

JIRA from time immemorial has always output

Unable to find 'webwork.multipart.saveDir' property setting

in its log. Can we get rid of it as a warn level log error.

This Google Search shows how people get worried about.

I have also seen support cases where people are querying about it so it costs us somehting to have this harmless but annoying message in place

The offending class is JiraServletDispatcher

File tempdir = (File) config.getServletContext().getAttribute("javax.servlet.context.tempdir");
log.warn("Unable to find 'webwork.multipart.saveDir' property setting. Defaulting to javax.servlet.context.tempdir");

It really should be a debug level warning.