UBI: get rid of ubi_ltree_slab

This slab cache is not really needed since the number of objects
is low and the constructor does not make much sense because we
allocate oblects when doint I/O, which is way slower then allocation.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

diff --cc drivers/mtd/ubi/build.c
Simple merge

diff --cc drivers/mtd/ubi/eba.c
Simple merge

diff --cc drivers/mtd/ubi/ubi.h
Simple merge