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

quak is pre-1.0 and keeps no handling of old data. The content cache no longer recognises or removes a live photo that an earlier quak version cached as one ZIP. A live-photo download no longer removes what was at its destination before renaming the image and video into place; the rename already replaces it. The README sentences and the tests about that old ZIP are gone. The two removed tests that also covered current behaviour are replaced by tests with no ZIP: the cache re-fetching a live photo recorded with no video, and the library telling the cache which files are live photos when it opens.

Model: opus-5-5

src/download/index.ts

@@ -361,8 +361,8 @@ const openPart = async (
 // written unpacked: each part is named `destination` with the extension
 // replaced by its own entry's, and the two must differ ignoring case. When the
 // file records a hash, `<imageHash>:<videoHash>` must match it, each over that
-// part's own bytes. Only then is whatever was at `destination` removed and the
-// image, then the video, renamed into place; on any failure neither is stored.
+// part's own bytes. Only then are the image, then the video, renamed into
+// place; on any failure neither is stored.
 //
 // The ZIP is chosen by its uploader and may expand enormously, so each part is
 // written as it decompresses and never held, and the ZIP is refused once the
@@ -486,7 +486,6 @@ const decryptLivePhoto = async (
             await part.handle.sync();
             await part.handle.close();
         }
-        await rm(destination, { force: true });
         await rename(image.tmpPath, path);
         try {
             await rename(video.tmpPath, videoPath);
@@ -570,8 +569,7 @@ const fetchAndDecrypt = async (
     }, api.getRetryOptions());
 
 // Write `file`'s original to `outPath`. A live photo is written as its image
-// and its video beside `outPath` instead, and whatever was at `outPath` is
-// removed (see `decryptLivePhoto`).
+// and its video beside `outPath` instead (see `decryptLivePhoto`).
 export const downloadFile = async (
     api: ApiClient,
     file: EnteFile,