cleanup: Improve robustness when multiple cleanups run concurrently

cleanup: Improve robustness when multiple cleanups run concurrently

The file count/size counters are now intentionally subtracted even if
there file to delete has disappeared since the final cache size
calculation will be incorrect if they aren’t. This can happen when there
are several parallel ongoing cleanups of the same subdirectory.

Also removed the “delete sibling files” logic; it’s unnecessary for all
siblings except .stderr since that’s the only file in a result that is
optional. Any other missing file will be detected by
get_file_from_cache.