mtd: nftlcore: remove set but rewrite variables

The value of lastEUN is not used at line 622, and will rewrite a new value
at line 541. so just remote the line at 622.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/1618363874-25113-1-git-send-email-tiantao6@hisilicon.com