The function nios2_check_interrupts)() looks only at CPU-internal
state; it belongs in target/nios2, not hw/nios2.  Move it into the
same file as its only caller, so it can just be local to that file.

This removes the only remaining code from cpu_pic.c, so we can delete
that file entirely.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20201129174022.26530-3-peter.maydell@linaro.org
Reviewed-by: Wentong Wu <wentong.wu@intel.com>
Tested-by: Wentong Wu <wentong.wu@intel.com>