includes: remove stale [smp|max]_cpus externs

Commit a5e0b331 removed these in favour of querying machine
properties. Remove the extern declarations as well.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190828165307.18321-6-alex.bennee@linaro.org
Cc: Like Xu <like.xu@linux.intel.com>
Message-Id: <20190711130546.18578-1-alex.bennee@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>