Avoid unnecessarily unwrapping stack_ref() after is_stack()

In a lot of cases, matching on `stack_ref()` also works.

src/shell/workspace.rs

@@ -713,8 +713,7 @@ impl Workspace {
         }
 
         let mapped = self.element_for_surface(surface)?;
-        let maybe_stack = mapped.stack_ref().filter(|s| s.len() > 1);
-        if let Some(stack) = maybe_stack
+        if let Some(stack) = mapped.stack_ref()
             && stack.len() > 1
         {
             let idx = stack.surfaces().position(|s| &s == surface);