You can't delete the file you have open from the cloud; if the nickname for your current file on the Connected Services tab in Settings is "Quicken 2007 File For Conversion" then you can't delete that account unless you close that file & open a different file. But the next time you open that file it will create a new cloud account for it.
As for what data is stored in the cloud, that depends. If you have Sync enabled on the Mobile, Web & Alerts tab then everything in your file is on the cloud. If you're using Quicken Connect as a connection method for any of your accounts, then everything about those accounts is in the cloud.
If you don't do either of those things, then I'm not sure what the cloud account is actually storing for that file, but there still is one. I'm assuming that it's just a stub account with no actual data in it, but I don't know that for certain.
@kohster "I really don't want to be storing a copy of my whole data file with Quicken."
I'm pretty sure that isn't happening. My main Quicken file is over 40MB compressed and over 150MB uncompressed. When I deleted the cloud account for that file & then reopened the file (creating a new cloud account) very little data got uploaded by Quicken (around 30KB). Even after several minutes only a few hundred KB got uploaded.
One other thing that apparently gets stored in the cloud account is E-Bill information, if you have any of those set up. And if you sign up for Quicken's Bill Pay service, there's going to be some info related to that getting stored in the cloud as well.
As for the data you uploaded when you did the file conversion, I don't know what ultimately happens to that data after the results are downloaded to your Mac.