soc: nordic: vpr: remove IRQ handling and enable RISCV_PRIVILEGED

IRQ handling functions are now in interrupt controller.
Enable necessary KConfigs to support CLIC properly.
A nice side effect of enabling RISCV_PRIVILIGED is that
`vector.S` is no longer necessary as common code handles
that.

Signed-off-by: Marcin Szymczyk <marcin.szymczyk@nordicsemi.no>