Remove unused callbacks (#11410)

This PR follows up #11314 (which removes some deprecated `callback`s)
removes the corresponding implements.

Release Notes:

- N/A

crates/gpui/src/platform/linux/x11/window.rs

@@ -147,7 +147,6 @@ pub struct Callbacks {
     input: Option<Box<dyn FnMut(PlatformInput) -> crate::DispatchEventResult>>,
     active_status_change: Option<Box<dyn FnMut(bool)>>,
     resize: Option<Box<dyn FnMut(Size<Pixels>, f32)>>,
-    fullscreen: Option<Box<dyn FnMut(bool)>>,
     moved: Option<Box<dyn FnMut()>>,
     should_close: Option<Box<dyn FnMut() -> bool>>,
     close: Option<Box<dyn FnOnce()>>,