Disable legacy supervised user creation by default

This has been disabled in release builds for a while, and might not work
anymore anyway.

Bug: none
Change-Id: I1cadea9d93944909958067eb75c3c64130da9d41
Reviewed-on: https://chromium-review.googlesource.com/959019Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542538}

chrome/common/chrome_features.cc

@@ -545,7 +545,7 @@ const base::Feature kSoundContentSetting{"SoundContentSetting",
 // Enables or disables the creation of (legacy) supervised users. Does not
 // affect existing supervised users.
 const base::Feature kSupervisedUserCreation{"SupervisedUserCreation",
-                                            base::FEATURE_ENABLED_BY_DEFAULT};
+                                            base::FEATURE_DISABLED_BY_DEFAULT};
 
 // Enables or disabled committed interstitials for Supervised User
 // interstitials.