tests: bluetooth: kconfig: Do not assign promptless BT_MESH_PROV

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

.

BT_MESH_PROV is already selected by BT_MESH_PB_ADV and BT_MESH_PB_GATT,
which these configuration files enable.

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