RISC-V: Don't include irq-riscv-intc.h

This file has never existed in the upstream kernel, but it's guarded by
an #ifdef that's also never existed in the upstream kernel.  As a part
of our interrupt controller refactoring this header is no longer
necessary, but this reference managed to sneak in anyway.

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