Remove the content cache's handling of an earlier version's live-photo ZIP (closes #151)

quak keeps no handling of old data. The content cache no longer checks
whether a live photo's file with no JSON file naming it is a ZIP an
earlier version cached, and no longer removes it; such a file is left
alone, as any other one already was. The README sentences and the tests
about that ZIP are gone, and one download test no longer names it in its
title. How a live photo is stored and served today is unchanged.

Model: opus-5-5

README.md

@@ -879,10 +879,7 @@ current account's records name.
 
 A live photo's original is cached as at its save path: its image and its video,
 each `originals/<fileID>.<ext>` with its own extension, and
-`originals/<fileID>.livephoto.json` naming them; the two are evicted together. A
-live photo that an earlier version cached as its ZIP is not served: the library
-removes the ZIP when it opens the cache, and fetches the two files when the
-photo is next read or precached.
+`originals/<fileID>.livephoto.json` naming them; the two are evicted together.
 
 A stored file appears only via an atomic temp-then-rename, so its presence means
 it is complete. Every downloaded original (by `quak get`, the cache, or