Remove the no-op --collection option from get and get-thumb (closes #241)

Both commands listed `--collection <id>` as accepted and ignored. It now
is refused as an unknown option, and the README no longer mentions it.

Model: opus-5-5

README.md

@@ -302,7 +302,7 @@ title, with unsafe characters replaced (see "File names from the server"). With
 original is kept in the cache as well.
 
 A file ID that no collection holds gives `File <id> not found`, and the exit
-status is 1. The option `--collection <id>` is accepted and ignored.
+status is 1.
 
 A live photo is a still image and a short video that Ente stores as one file.
 `get` writes both, each named after the title with its own extension, as Ente's