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/dissimilaroriginwindow.rs

@@ -64,7 +64,6 @@ impl DissimilarOriginWindow {
                 // FIXME(nox): The microtask queue is probably not important
                 // here, but this whole DOM interface is a hack anyway.
                 global_to_clone_from.microtask_queue().clone(),
-                global_to_clone_from.is_headless(),
                 global_to_clone_from.get_user_agent(),
                 #[cfg(feature = "webgpu")]
                 global_to_clone_from.wgpu_id_hub(),