Revert "[newauth] Enable by default"

This reverts commit 2e9d2d7ebe.

Reason for revert: this is the possible culprit for b/408427309

Original change's description:
> [newauth] Enable by default
>
> Bug: 404613530
> Change-Id: If0a214fe074d7e7591b5d37417ca447525c983b6
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6418563
> Reviewed-by: Yiwei Zhang <yiwzhang@google.com>
> Commit-Queue: Allen Li <ayatane@chromium.org>

Bug: 404613530, 408427309
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: Ib919b2f3d852709c0e80622b4ad97327ac2ea5bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6434729
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Scott Lee <ddoman@chromium.org>
Commit-Queue: Scott Lee <ddoman@chromium.org>

newauth.py

@@ -13,7 +13,7 @@ import scm
 
 def Enabled() -> bool:
     """Returns True if new auth stack is enabled."""
-    if ExplicitlyDisabled():
+    if not EnabledInConfig():
         return False
     if _HasGitcookies():
         _PrintGitcookiesWarning()