mtd: mtd_oobtest: Remove redundant assignment to err

Variable err is set to zero but this value is never read as it is
overwritten with a new value later on, hence it is a redundant
assignment and can be removed.

Cleans up the following clang-analyzer warning:

drivers/mtd/tests/oobtest.c:626:4: warning: Value stored to 'err' is
never read [clang-analyzer-deadcode.DeadStores].

drivers/mtd/tests/oobtest.c:603:4: warning: Value stored to 'err' is
never read [clang-analyzer-deadcode.DeadStores].

drivers/mtd/tests/oobtest.c:579:4: warning: Value stored to 'err' is
never read [clang-analyzer-deadcode.DeadStores].

drivers/mtd/tests/oobtest.c:556:4: warning: Value stored to 'err' is
never read [clang-analyzer-deadcode.DeadStores].

drivers/mtd/tests/oobtest.c:532:3: warning: Value stored to 'err' is
never read [clang-analyzer-deadcode.DeadStores].

drivers/mtd/tests/oobtest.c:509:3: warning: Value stored to 'err' is
never read [clang-analyzer-deadcode.DeadStores].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/1619346498-43617-1-git-send-email-jiapeng.chong@linux.alibaba.com