ui/cocoa: Fix switched_to_fullscreen warning

ui/cocoa: Fix switched_to_fullscreen warning

I noticed this error while building QEMU on Mac OS X:

    [1040/1660] Compiling Objective-C object libcommon.fa.p/ui_cocoa.m.o
    ../ui/cocoa.m:803:17: warning: variable 'switched_to_fullscreen' set but not used [-Wunused-but-set-variable]
        static bool switched_to_fullscreen = false;
                    ^
    1 warning generated.

I think the behavior is fine if you remove "switched_to_fullscreen", I can
still switch in and out of mouse grabbed mode and fullscreen mode with this
change, and Command keycodes will only be passed to the guest if the mouse
is grabbed, which I think is the right behavior. I'm not sure why a static
piece of state was needed to handle that in the first place. Perhaps the
refactoring of the flags-state-change fixed that by toggling the Command
keycode on.

I tested this with an Ubuntu core image on macOS 12.4

    wget https://cdimage.ubuntu.com/ubuntu-core/18/stable/current/ubuntu-core-18-i386.img.xz
    xz -d ubuntu-core-18-i386.img.xz
    qemu-system-x86_64 -drive file=ubuntu-core-18.i386.img,format=raw

Fixes: 6d73bb643aa7 ("ui/cocoa: Clear modifiers whenever possible")
Signed-off-by: Peter Delevoryas <peter@pjd.dev>
Reviewed-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220702044304.90553-1-peter@pjd.dev>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>