Remove leftover SCREEN_OOBE_WELCOME

SCREEN_OOBE_WELCOME was (mostly) removed in
https://chromium-review.googlesource.com/c/chromium/src/+/1520310/8/ui/login/display_manager.js
except for one occurrence. This occurrence prevents the ctrl+alt+E key
combination to work on network and eula screen and also produces error
log output. This CL removes the remaining occurrence. This change is
safe since its old behavior was already replaced by
attributes.postponeEnrollmentAllowed, which is true for the welcome
screen.

Bug: 965391
Change-Id: Ie8d80c25b8c240f1f5d36ab7c28d15143e48474d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1640407
Auto-Submit: Alexander Hendrich <hendrich@chromium.org>
Commit-Queue: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667243}

ui/login/display_manager.js

@@ -398,7 +398,6 @@ cr.define('cr.ui.login', function() {
             currentStepId == SCREEN_ACCOUNT_PICKER) {
           chrome.send('toggleEnrollmentScreen');
         } else if (attributes.postponeEnrollmentAllowed ||
-            currentStepId == SCREEN_OOBE_WELCOME ||
             currentStepId == SCREEN_OOBE_NETWORK ||
             currentStepId == SCREEN_OOBE_EULA) {
           // In this case update check will be skipped and OOBE will