servoshell: Move headless setting to ServoShellPreferences (#35377)

This is only used in servoshell, even though it was plumbed through
script previously. It's just about how the `RenderingContext` is set up,
which is something managed entirely outside of servo itself.

In addition, make the name of `servo_shell_preferences` in `app.rs` more
consistent with the rest of the codebase.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/window.rs

@@ -2778,7 +2778,6 @@ impl Window {
         unminify_css: bool,
         local_script_source: Option<String>,
         userscripts_path: Option<String>,
-        is_headless: bool,
         replace_surrogates: bool,
         user_agent: Cow<'static, str>,
         player_context: WindowGLContext,
@@ -2807,7 +2806,6 @@ impl Window {
                 origin,
                 Some(creator_url),
                 microtask_queue,
-                is_headless,
                 user_agent,
                 #[cfg(feature = "webgpu")]
                 gpu_id_hub,