boards: stm32: kconfig: Do not assign promptless ENTROPY_HAS_DRIVER

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

The removed lines may look like just comments, but they're equivalent to
CONFIG_ENTROPY_HAS_DRIVER=n. Search for "is not set" in
https://docs.zephyrproject.org/latest/guides/kconfig/setting.html for an
explanation.

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