chore: remove unused WinFrameView::kInactiveTitlebarFeatureAlpha (#43475)

added in Aug 2021 (41646d11, #29600) but never used

Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Charles Kerr <charles@charleskerr.com>

shell/browser/ui/views/win_frame_view.h

@@ -28,10 +28,6 @@ class WinFrameView : public FramelessView {
   void Init(NativeWindowViews* window, views::Widget* frame) override;
   void InvalidateCaptionButtons() override;
 
-  // Alpha to use for features in the titlebar (the window title and caption
-  // buttons) when the window is inactive. They are opaque when active.
-  static constexpr SkAlpha kInactiveTitlebarFeatureAlpha = 0x66;
-
   SkColor GetReadableFeatureColor(SkColor background_color);
 
   // views::NonClientFrameView: