mu4e: restore mu4e-index-cleanup t

Let's go back to the old default.

mu4e/mu4e.texi

@@ -584,10 +584,12 @@ messages, even if other programs have modified the Maildir.
 
 The downside of this thoroughness (which is the default) is that it is
 relatively slow, something that can be noticeable with large e-mail
-corpa. For a faster approach, you can use the following:
+corpa on slow file-systems. For a faster approach, you can use the
+following:
 
 @lisp
 (setq
+  mu4e-index-cleanup nil      ;; don't do a full cleanup check
   mu4e-index-lazy-check t)    ;; don't consider up-to-date dirs
 @end lisp