includes: remove stale [smp|max]_cpus externs

includes: remove stale [smp|max]_cpus externs

Commit a5e0b3311 removed these in favour of querying machine
properties. Remove the extern declarations as well.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Like Xu <like.xu@linux.intel.com>
Message-Id: <20190711130546.18578-1-alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Like Xu <like.xu@linux.intel.com>
Fixes: a5e0b331193a ("vl.c: Replace smp global variables with smp machine properties")
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>