Don't double-count the 'node stat' metric.

This is already done in RealDiskInterface::Stat() itself,
and removes a confusing duplicate line in `-d stats` output,
e.g.:

BEFORE:
  metric                  count   avg (us)        total (ms)
  ...
  node stat               119145  3.0             355.2
  node stat               270673  3.1             834.9

AFTER:
  metric                  count   avg (us)        total (ms)
  ...
  node stat               270673  2.9             774.0

src/graph.cc

@@ -32,7 +32,6 @@
 using namespace std;
 
 bool Node::Stat(DiskInterface* disk_interface, string* err) {
-  METRIC_RECORD("node stat");
   mtime_ = disk_interface->Stat(path_, err);
   if (mtime_ == -1) {
     return false;