Removed globally set mail_uid and mail_gid

Co-Authored-By: Antoine Nguyen <tonio@ngyn.org>

modoboa_installer/scripts/files/dovecot/conf.d/10-mail.conf

@@ -104,8 +104,8 @@ namespace inbox {
 # System user and group used to access mails. If you use multiple, userdb
 # can override these by returning uid or gid fields. You can use either numbers
 # or names. <doc/wiki/UserIds.txt>
-mail_uid = %{mailboxes_owner_uid}
-mail_gid = %{mailboxes_owner_gid}
+#mail_uid =
+#mail_gid =
 
 # Group to enable temporarily for privileged operations. Currently this is
 # used only with INBOX when either its initial creation or dotlocking fails.