Removed Win10 specific welcome UI flow.

This flow has been removed from Chromium.

Chromium changes:

https://bugs.chromium.org/p/chromium/issues/detail?id=956680

https://chromium.googlesource.com/chromium/src/+/afd6d11e54a3682cf1e4d0476e598eb234488246

commit afd6d11e54a3682cf1e4d0476e598eb234488246
Author: Hector Carmona <hcarmona@chromium.org>
Date:   Mon Jun 24 21:07:42 2019 +0000

    Navi: Delete entry point for showing chrome://welcome-win10

    Bug: 956680

https://chromium.googlesource.com/chromium/src/+/9359944557ddce6a63516df681eb38e0a70829df

commit 9359944557ddce6a63516df681eb38e0a70829df
Author: Hector Carmona <hcarmona@chromium.org>
Date:   Wed Jun 26 17:56:25 2019 +0000

    Navi: Deprecate kHasSeenWin10PromoPage pref.

    Bug: 956680

browser/brave_content_browser_client.cc

@@ -325,8 +325,7 @@ bool BraveContentBrowserClient::HandleURLOverrideRewrite(GURL* url,
   }
 
   // no special win10 welcome page
-  if (url->host() == chrome::kChromeUIWelcomeWin10Host ||
-      url->host() == chrome::kChromeUIWelcomeHost) {
+  if (url->host() == chrome::kChromeUIWelcomeHost) {
     *url = GURL(chrome::kChromeUIWelcomeURL);
     return true;
   }

browser/ui/webui/brave_welcome_ui.cc

@@ -75,11 +75,6 @@ BraveWelcomeUI::BraveWelcomeUI(content::WebUI* web_ui, const std::string& name)
       std::make_unique<settings::SearchEnginesHandler>(profile));
 
   profile->GetPrefs()->SetBoolean(prefs::kHasSeenWelcomePage, true);
-#if defined(OS_WIN)
-  g_brave_browser_process->local_state()->SetBoolean(
-      prefs::kHasSeenWin10PromoPage,
-      true);
-#endif
 }
 
 BraveWelcomeUI::~BraveWelcomeUI() {

chromium_src/chrome/browser/browser_about_handler.cc

@@ -14,10 +14,6 @@ bool FixupBrowserAboutURL(GURL* url,
                           content::BrowserContext* browser_context) {
   bool result = FixupBrowserAboutURL_ChromiumImpl(url, browser_context);
 
-  // no special win10 welcome page
-  if (url->host() == chrome::kChromeUIWelcomeWin10Host)
-    *url = GURL(chrome::kChromeUIWelcomeURL);
-
   // redirect sync-internals
   if (url->host() == chrome::kChromeUISyncInternalsHost) {
     GURL::Replacements replacements;