remove 'old' FAILED files in the next acquire call

If apt renames a file to .FAILED it leaves its namespace and is never
touched again – expect since 1.1~exp4 in which "apt clean" will remove
those files. The usefulness of these files rapidly degrades if you don't
keep the update log itself (together with debug output in the best case)
through and on 99% of all system they will be kept around forever just
to collect dust over time and eat up space.

With this commit an update call will remove all FAILED files of previous
runs, so that the FAILED files you have on disk are always only the ones
related to the last apt run stopping apt from hoarding files.

Closes: 846476

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