[Brian]

That folder doesn't exist on my machine, so I think those files are left over from the web interface we started work on back in the pre-OmniFocus 1.0 days - it's been discontinued for a couple of years now.

My guess is that the web interface was turned on at some point, and those logs accumulated between then and now. You can delete them.

If this file exists on your machine, you'll want to delete it and reboot the machine:
~/Library/LaunchAgents/com.omnigroup.omnifocus.web_interface.plist
That'll prevent the server running the web interface from launching again.
(If the file doesn't exist, I suspect the server isn't running any more, but those log files just got left around.)