ui/cocoa: Drop workarounds for pre-10.12 OSX

ui/cocoa: Drop workarounds for pre-10.12 OSX

Our official OSX support policy covers the last two released versions.
Currently that is 10.14 and 10.15.  We also may work on older versions, but
don't guarantee it.

In commit 50290c002c045280f8d in mid-2019 we introduced some uses of
CLOCK_MONOTONIC which incidentally broke compilation for pre-10.12 OSX
versions (see LP:1861551). We don't intend to fix that, so we might
as well drop the code in ui/cocoa.m which caters for pre-10.12
versions as well. (For reference, 10.11 fell out of Apple extended
security support in September 2018.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200201170534.22123-1-peter.maydell@linaro.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>