generation: Remove usedless keys in metadata.

For an explanation of the problem those keys was working around and a
resolution thereof, see
https://www.erp5.com/group_section/forum/Need-help-to-fix-shacache-upload-SEiYkNyXWq/view?list_start=8&reset=1#2061505002

generation/40upload-image.sh

@@ -30,9 +30,7 @@ json_metadata="{ \
 	\"flb-version\": \"$(version_to_list $FLB_VERSION)\", \
 	\"plg-version\": \"$(version_to_list $PLG_VERSION)\", \
 	\"timestamp\": $(date +%s), \
-	\"comment\": \"$hash_comment\", \
-	\"file\": \"notused\", \
-	\"urlmd5\": \"notused\" \
+	\"comment\": \"$hash_comment\" \
 }"
 echo "$json_metadata" > "$metadata_file"
 networkcache-upload --conf ../tls-access-to-shacache/test.cfg --id river --file "$uefi_image_name" --metadata "$metadata_file"