Retire UseWindowsSystemColors runtime flag

The Blink flag UseWindowsSystemColors has been stable since 2019-10.

Bug: 970285
Change-Id: Ifee7d8ad4fcafd30bfd518250e729f2e650d463f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2208775Reviewed-by: Alison Maher <almaher@microsoft.com>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Auto-Submit: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770736}

third_party/blink/renderer/core/layout/layout_theme_win.cc

@@ -22,10 +22,6 @@ LayoutTheme& LayoutTheme::NativeTheme() {
 
 Color LayoutThemeWin::SystemColor(CSSValueID css_value_id,
                                   WebColorScheme color_scheme) const {
-  if (!RuntimeEnabledFeatures::UseWindowsSystemColorsEnabled()) {
-    return LayoutThemeDefault::SystemColor(css_value_id, color_scheme);
-  }
-
   blink::WebThemeEngine::SystemThemeColor theme_color;
   switch (css_value_id) {
     case CSSValueID::kActivetext:

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1811,10 +1811,6 @@
       name: "UserAgentClientHint",
       status: "stable"
     },
-    {
-      name: "UseWindowsSystemColors",
-      status: "stable",
-    },
     {
       name: "V8IdleTasks",
     },