Kill jffs2-user.h

This file does not define any kernel-userspace API, all
it does it defines few helpers for userspace. Instead,
userspace should have a private copy of this file.

The main (if not the only) user is the mtd-utils package, but
it already has a private copy of this file.

This patch also removes references to 'jffs2-user.h' from
'Kbuild' and MAINTAINERS' files.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>