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/workerglobalscope.rs

@@ -79,7 +79,6 @@ pub(crate) fn prepare_workerscope_init(
         pipeline_id: global.pipeline_id(),
         origin: global.origin().immutable().clone(),
         creation_url: global.creation_url().clone(),
-        is_headless: global.is_headless(),
         user_agent: global.get_user_agent(),
         inherited_secure_context: Some(global.is_secure_context()),
     };
@@ -165,7 +164,6 @@ impl WorkerGlobalScope {
                 MutableOrigin::new(init.origin),
                 init.creation_url,
                 runtime.microtask_queue.clone(),
-                init.is_headless,
                 init.user_agent,
                 #[cfg(feature = "webgpu")]
                 gpu_id_hub,