osdep.h: Clean up no-longer-needed back-compat for macOS 10

osdep.h: Clean up no-longer-needed back-compat for macOS 10

Our official support policy only supports the most recent two
versions of macOS (currently macOS 13 Ventura and macOS 14 Sonoma),
and we already have code that assumes at least macOS 12 Monterey or
better.  In commit 2d27c91e2b72ac7 we dropped some of the back-compat
code for older macOS versions, but missed the guard in osdep.h that
is providing a fallback for macOS 10 and earlier.

Simplify the ifdef to the "ifdef __APPLE__" that we use elsewhere for
"is this macOS?".

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240730095939.2781172-1-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>