irqchip/riscv-intc: remove user selectability of RISCV_INTC

Since commit e71ee06e ("RISC-V: Force select RISCV_INTC for
CONFIG_RISCV") the driver has been enabled at the arch level - and is
mandatory anyway. There's no point exposing this as a choice to users,
so stop bothering.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221118104300.85016-3-conor@kernel.org


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>