build_telemetry: Do not print warning message for cipd auth-info error

The warning message is always printed for non-Googlers.
https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5842066/comments/c124a640_3e9ee3ea

Change-Id: I9be51813e7531adcdeed6e2f7446aedc8700b880
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5848451
Commit-Queue: Junji Watanabe <jwata@google.com>
Reviewed-by: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Auto-Submit: Junji Watanabe <jwata@google.com>

build_telemetry.py

@@ -154,11 +154,10 @@ def check_auth():
             "cipd auth-info --json-output -",
             text=True,
             shell=True,
+            stderr=subprocess.DEVNULL,
             timeout=3,
         )
     except Exception as e:
-        print("WARNING: depot_tools.build_telemetry: "
-              f"failed to get auth info: {e}")
         return {}
     try:
         return json.loads(out)