Fix mu4e’s completing read setting

Vertico doesn’t have such a function but hooks into the built-in one.

configuration.org

@@ -2128,7 +2128,6 @@ accompanying function will be added to ~mu4e-view-mode-hook~.
                      (message-sendmail-extra-arguments . ("--account=private" "--read-envelope-from")))))
         org-mu4e-link-query-in-headers-mode nil)
   :custom
-  (mu4e-completing-read-function vertico-completing-read)
   (mu4e-context-policy 'pick-first)
   (mu4e-confirm-quit nil)
   (mail-user-agent 'sendmail-user-agent)