mtd: spi-nor: atmel-quadspi: Remove unused code from atmel-quadspi driver

Code used for previous interface is no longer needed.
This change just removes obsolete code.

Suggested-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Piotr Bugalski <bugalski.piotr@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>