s3: add progress detail

this is a bit of a hack and we should add another way to set the progress
(maybe via backups.setProgress or via a progress callback). this is because
some methods like removeDir can be called from backuptask and from box code.

src/progress.js

@@ -52,10 +52,6 @@ function clear(tag) {
     debug('clearing %s', tag);
 }
 
-function get(tag) {
-    return progress[tag];
-}
-
 function getAll() {
     return progress;
 }