windows: Remove unnecessary Send and Sync implementations (#14659)

After a update to `windows-rs 0.57`, these two implementations are no
longer needed.

Release Notes:

- N/A

crates/gpui/src/platform/windows/dispatcher.rs

@@ -31,9 +31,6 @@ pub(crate) struct WindowsDispatcher {
     main_thread_id: ThreadId,
 }
 
-unsafe impl Send for WindowsDispatcher {}
-unsafe impl Sync for WindowsDispatcher {}
-
 impl WindowsDispatcher {
     pub(crate) fn new() -> Self {
         let controller = unsafe {