mtd: nand: vf610_nfc: remove unused function

The function count_written_bits has been replaced by the generic
nand_check_erased_ecc_chunk() function with commit 48c25cf4
("mtd: nand: vf610_nfc: use nand_check_erased_ecc_chunk() helper").
Remove the unused function.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>