Remove StatIfNecessary call that is never necessary

Remove StatIfNecessary call that is never necessary

The call to StatIfNecessary in DependencyScan::RecomputeOutputsDirty was
added by commit v1.4.0^2~7^2~1 (Share more code between CleanNode() and
RecomputeDirty(), 2013-09-02) while consolidating code paths.  However,
it was needed only when called from RecomputeDirty because prior to
refactoring the CleanNode code path did not call it.

Later commit v1.6.0^2~46^2 (Let DependencyScan::RecomputeDirty() work
correclty with cyclic graphs, 2014-12-07) added back to RecomputeDirty a
loop over outputs that calls StatIfNecessary.  Therefore
RecomputeOutputsDirty no longer needs to call StatIfNecessary for either
of its own callers.