mtdblock: Update old JFFS2 mention in Kconfig

JFFS2 can be mounted without 'mtdblock' since
a really, really long time. Some git-log
archaeology shows that in 2006 it was possible
to use 'root=' to mount a JFFS2 rootfs:

  commit e9482b43
  Author: Joern Engel <joern@wh.fh-wedel.de>
  Date:   Tue May 30 14:25:46 2006 +0200

      [MTD] Allow alternate JFFS2 mount variant for root filesystem.

      With this patch, "root=mtd3" and "root=mtd:foo" work for a JFFS2 rootfs.

However, there are still plenty of tutorials that mention
mtdblock, so users are still taking this route. Update the Kconfig
to reflect this is no longer needed.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210801234509.18774-6-ezequiel@collabora.com