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

test/download/download.test.ts

@@ -1883,7 +1883,7 @@ describe("downloadFile live photos", () => {
         expect(readdirSync(t.dir).sort()).toEqual(["f.JPG", "f.bin"]);
     });
 
-    it("replaces what was at the destination, such as an earlier ZIP of the two", async () => {
+    it("replaces what was at the destination", async () => {
         const t = setup(livePhotoZip(), livePhoto);
         writeFileSync(t.outPath, livePhotoZip());