mtd: rawnand: ams-delta: Drop board specific partition info

Now as we support fetching partition info from device platform data and
the Amstrad Delta board file provides that info, drop it from the
driver code.

v2: rebase on top of gpio_nand_platdata extension

Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200212003929.6682-5-jmkrzyszt@gmail.com