tests: userspace: Do not assign promptless THREAD_USERSPACE_LOCAL_DATA

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

.

This symbol should already be getting enabled if CONFIG_USERSPACE is
enabled, because CONFIG_ERRNO is default y and has

    select THREAD_USERSPACE_LOCAL_DATA if USERSPACE

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