drivers: flash: Remove redundant FLASH dep. from FLASH_NATIVE_POSIX

FLASH_NATIVE_POSIX is defined in drivers/flash/Kconfig.native_posix,
which is source'd within an 'if FLASH' in drivers/flash/Kconfig.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>