Remove a NOTIMPLEMENTED in TestingBrowserProcess to cut down on log spam

For certain tests (such as ProfileResetterTest.ResetExtensionsByDisabling),
this was logging 2-6 times.

Bug: none
Change-Id: Ib83a0b9bf8dde67a61288acb123f7527261be37e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1762557
Commit-Queue: Evan Stade <estade@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Auto-Submit: Evan Stade <estade@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688657}

chrome/test/base/testing_browser_process.cc

@@ -285,7 +285,6 @@ NotificationUIManager* TestingBrowserProcess::notification_ui_manager() {
     notification_ui_manager_ = NotificationUIManager::Create();
   return notification_ui_manager_.get();
 #else
-  NOTIMPLEMENTED();
   return nullptr;
 #endif
 }