ubi: wl: Remove set but not used variable 'prev_e'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/mtd/ubi/wl.c: In function 'find_wl_entry':
drivers/mtd/ubi/wl.c:322:27: warning:
 variable 'prev_e' set but not used [-Wunused-but-set-variable]

It's not used any more now, so remove it.

Fixes: f9c34bb5 ("ubi: Fix producing anchor PEBs")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>