It keeps getting defaulted to this. I have no idea why because it is not even listed as a workflow constant. I just tested it and I am unable to blank the credential fields out. Upon save it pushed these credentials back in there until I type something else in.
Odd. I can't seem to reproduce the issue. Definitely strange behaviour.
This is what the Help says, but I am able to leave this blank and publish it that way.
If the destination library is on a remote SharePoint environment, credentials with access to upload files are required. For copying files to a SharePoint site in the same environment as the workflow, the workflow automatically uses permissions with global SharePoint access if no other credentials are specified. To copy the item as a specific account in the local farm, enter the user credentials.