include: hw: remove ibex_plic.h

This patch removes the left-over/unused `ibex_plic.h` file. Previously
used by opentitan, which now follows the RISC-V standard and uses the
SiFivePlicState.

Fixes: 434e7e0 ("hw/intc: Remove the Ibex PLIC")
Signed-off-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20220121055005.3159846-1-alistair.francis@opensource.wdc.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>