Update error message now that /proc/cpuinfo is no longer in use (#1917)

c24774dc4f4402c3ad150363321cc972ed2669e7 removed using /proc/cpuinfo
so no longer mention it in the error message.

src/sysinfo.cc

diff --git a/src/sysinfo.cc b/src/sysinfo.cc
index eddd430..2787e87 100644
--- a/src/sysinfo.cc
+++ b/src/sysinfo.cc
@@ -515,8 +515,7 @@
 int GetNumCPUs() {
   int num_cpus = GetNumCPUsImpl();
   if (num_cpus < 1) {
-    std::cerr << "Unable to extract number of CPUs.  If your platform uses "
-                 "/proc/cpuinfo, custom support may need to be added.\n";
+    std::cerr << "Unable to extract number of CPUs.\n";
     /* There is at least one CPU which we run on. */
     num_cpus = 1;
   }