HTTP: Shut up file delete error message

HTTP: Shut up file delete error message

For #67.

When the file didn't need to be downloaded, because the IMS header did
its job, nothing needs to be deleted. So Fort shouldn't be complaining.

Like the previous commit, this is not a great solution, because IMS is
not the only trigger of file deletes, and the error message might be
helpful in other cases. Then again, I don't really agree with this eager
repository cleaning technique; it complicates debugging.

The proper solution is a WIP in the rrdp-refactor branch.