mtd: nand: drop EXPORT_SYMBOL_GPL for nanddev_erase()

This function is only used within this module, so it is no longer
necessary to use EXPORT_SYMBOL_GPL().

This patch parallels the work done in the following patch:
https://lore.kernel.org/linux-mtd/20221018170205.1733958-1-dario.binacchi@amarulasolutions.com



Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-By: Michael Trimarchi <michael@amarulasolutions.com>
Link: https://lore.kernel.org/all/20221108090719.3631621-1-dario.binacchi@amarulasolutions.com