boards: cc2650_sensortag: Remove TRNG and PINMUX prio Kconfig settings

As mentioned in
https://github.com/zephyrproject-rtos/zephyr/issues/7873

, these Kconfig
symbols never existed. Remove the assignments to them.

Keep the dead code that's guarded by them for a while. I don't know if
this board is used.

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