tests: Clear SPI_*_IRQ_PRI from tests and samples

Remove SPI_*_IRQ_PRI from tests and samples.
Using DT to get the *_IRQ_PRI, we can't
override it using Kconfig.

If needed, use a BOARD_NAME.overlay file to
override default values.

Signed-off-by: Yannis Damigos <giannis.damigos@gmail.com>