Remove unnecessary unsafe impls for WinitWindows on Wasm (#11270)

# Objective

In the past `winit::window::Window` was not Send + Sync on web.
rust-windowing/winit#2834 made
`winit::window::Window` Sync + Send so Bevy's `unsafe impl` is no longer
necessary.

## Solution

Remove the unsafe impls.

crates/bevy_winit/src/winit_windows.rs

@@ -356,9 +356,3 @@ pub fn winit_window_position(
         }
     }
 }
-
-// WARNING: this only works under the assumption that wasm runtime is single threaded
-#[cfg(target_arch = "wasm32")]
-unsafe impl Send for WinitWindows {}
-#[cfg(target_arch = "wasm32")]
-unsafe impl Sync for WinitWindows {}