mtd: ssfdc: Remove an unused variable

Since its introduction the driver was declaring a "usecount" variable,
but nobody ever used it upstream.

This was spot while grepping for usecount through mtd/ for other
reasons.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20231205075431.13401-1-miquel.raynal@bootlin.com