boards: kconfig: Do not assign promptless SERIAL_HAS_DRIVER

Assignments have no effect on promptless symbols. Flagged by
https://github.com/zephyrproject-rtos/zephyr/pull/20742

.

This symbol is enabled through being selected by other symbols.

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