samples: sht3xd: kconfig: Do not assign promptless SHT3XD_TRIGGER

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

.

SHT3XD_TRIGGER is already being selected by
SHT3XD_TRIGGER_GLOBAL_THREAD, which is enabled in the same configuration
file.

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