Don't allow strong view handles to be read/updated with an AsyncAppContext

This avoids an invitation to hold strong view handles across async await
points, which is a common source of leaks.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

crates/workspace/src/dock.rs

@@ -811,8 +811,6 @@ mod tests {
     }
 
     impl BorrowWindowContext for DockTestContext<'_> {
-        type ReturnValue<T> = T;
-
         fn read_with<T, F: FnOnce(&WindowContext) -> T>(&self, window_id: usize, f: F) -> T {
             BorrowWindowContext::read_with(self.cx, window_id, f)
         }