don't move failed pdiff indexes out of partial

The comment says this is intended, but looking at the history reveals
that the comment comes from a different era. Nowadays we don't really
need it anymore (and even back then it was disputeable) as we haven't
used that file for our update in the end and nothing really needs this
file after the update.

Triggered is this by 188f297a2af4c15cb1d502360d1e478644b5b810 which
moves various error conditions forward including this code expecting the
file to exist – but it doesn't need to as download could have failed.
We could fix that by simple checking if the file exists and only stage
it if it does, but instead we don't stage it and instead even rename it
out of the way with our conventional FAILED name (if it exists).

That restores support for partial mirrors (= in this case mirrors which
don't ship pdiff files). Note that apt heals itself even if only such a
mirror is used as the update is successful even if that error is shown.

Closes: 869425

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