Remove support for CLOCK_MONOTONIC not being defined

Remove support for CLOCK_MONOTONIC not being defined

Some older parts of QEMU's codebase assume that CLOCK_MONOTONIC
might not be defined by the host OS, and have workarounds to
deal with this. However, more recently (notably in commit
50290c002c045280f8d for qemu-img in mid-2019, but also much
earlier in 2011 in commit 22795174a37e0 for ui/spice-display.c)
we've written code that assumes CLOCK_MONOTONIC is always
defined. The only host OS anybody's ever noticed this on
is OSX 10.11 and earlier, which we don't support.

So we can assume that all our host OSes have the #define,
and we can remove some now-unnecessary ifdefs.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200201172252.6605-1-peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>