riscv: plic: Remove unused interrupt functions

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Jonathan Behrens <fintelia@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Chih-Min Chao <chihmin.chao@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>