mtd: mtdconcat: map: remove redundant assignment to variable 'size'

Variable 'size' is being assigned the value zero that will never be
read. The assignment is redundant and can be removed.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200910154451.752569-1-colin.king@canonical.com