UBI: remove dead code

Thanks to new gcc 4.6 for issuing the following warning:

drivers/mtd/ubi/vtbl.c: In function ‘create_vtbl’:
drivers/mtd/ubi/vtbl.c:311:33: warning: variable ‘old_seb’ set but not used [-Wunused-but-set-variable]

This patch removes some dead code and fixes the warning.

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