ui/cocoa.m: remove open dialog code

ui/cocoa.m: remove open dialog code

Removes the open dialog code that runs when no arguments are supplied with QEMU.
Not everyone needs a hard drive or cdrom to boot their target. A user might only
need to use their target's bios to do work. With that said, this patch removes
the unneeded open dialog code.

Signed-off-by: John Arbuckle <programmingkidx@gmail.com>
Message-id: 33856864-321C-4367-9170-FB0BF81E789B@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>