Changing a file fetcher upload from stand alone to attached to node will cause initial upload to appear on form

This is a minor bug, but should be fixed.

To reproduce,
1) create a new importer not attached to a node.
2) do a test upload
3) change this importer so it's now attached to a node
4) create a new feed using the node add and you'll see the original file uploaded on your first test upload as a default

The fix is to remove any records from the feeds_source table with an nid of 0 and the feed id that you want to fix. There may be some other cleanup that should/can be done on other tables as well.