mu4e: restore mu4e-index-cleanup t

Let's go back to the old default.

mu4e/mu4e-vars.el

@@ -96,19 +96,17 @@ This depends on the `mu4e-get-mail-command' exit code."
   :group 'mu4e
   :safe 'booleanp)
 
-(defcustom mu4e-index-cleanup nil
+(defcustom mu4e-index-cleanup t
   "Whether to run a cleanup phase after indexing.
 
 That is, validate that each message in the message store has a
 corresponding message file in the filesystem.
 
 Having this option as t ensures that no non-existing messages are
-shown but can also be quite slow with large message stores, and
-especially with slow filesystems."
+shown but can slow with large message stores on slow file-systems."
   :type 'boolean
   :group 'mu4e
-  :safe 'booleanp
-  :version "1.3.9")
+  :safe 'booleanp)
 
 (defcustom mu4e-index-lazy-check nil
   "Whether to only use a 'lazy check' during reindexing.

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