sync: Silence 'not found in manifest' message

This can potentially show up when sync'ing projects with submodules
that are not declared in the manifest as well as the internal
'.repo/repo' project, which is likely not desirable from a user
standpoint.

Change-Id: I93d7fcd6e3fd1818357ea4537882a864dea9942c
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/355920
Reviewed-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Michael Kelly <mkelly@arista.com>
diff --git a/subcmds/sync.py b/subcmds/sync.py
index 8db429b..1c38d26 100644
--- a/subcmds/sync.py
+++ b/subcmds/sync.py
@@ -772,7 +772,7 @@
       # We do not support switching between the options.  The environment
       # variable is present for testing and migration only.
       return not project.UseAlternates
-    print(f'\r{relpath}: project not found in manifest.', file=sys.stderr)
+
     return False
 
   def _SetPreciousObjectsState(self, project: Project, opt):