[JFFS2] remove junk prototypes

'rb_prev()', 'rb_next()' and 'rb_replace_node()' are declared in
include/linux/rbtree.h, no need for JFFS2 to re-declare them. I
believe these are left-overs from the old days when the common
RB tree code did not have those call and JFFS2 had private
implementation.

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