WindowServer+LibGUI: Yank out window-global opacity

From what I can tell, this facility was added to WSWindow/GWindow in
2019 in 9b71307. I only found a single place in the codebase still using
this facility: `WindowServer::Menu::start_activation_animation()`. A
subtle fade-out animation that happens when a menu item is selected, and
the menu disappears.
I think our compositing facilities have improved enough to make this
facility redundant. The remaining use mentioned above was ported to just
directly blit the fade-out animation instead of requesting it from
WindowServer.

Userland/Libraries/LibGUI/Window.cpp

@@ -159,7 +159,6 @@ void Window::show()
         m_fullscreen,
         m_frameless,
         m_forced_shadow,
-        m_opacity_when_windowless,
         m_alpha_hit_threshold,
         m_base_size,
         m_size_increment,
@@ -895,14 +894,6 @@ void Window::set_double_buffering_enabled(bool value)
     m_double_buffering_enabled = value;
 }
 
-void Window::set_opacity(float opacity)
-{
-    m_opacity_when_windowless = opacity;
-    if (!is_visible())
-        return;
-    ConnectionToWindowServer::the().async_set_window_opacity(m_window_id, opacity);
-}
-
 void Window::set_alpha_hit_threshold(float threshold)
 {
     if (threshold < 0.0f)