samples: drivers: spi_flash: Remove unnecessary Kconfig option changes

It is no longer needed to enable a specific flash driver in the sample
configuration. An appropriate driver (together with its dependecies,
like the SPI bus) for a given target will be enabled automatically
based on what is enabled in DTS.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>