grabs/moving: Call cleanup_drag() unconditionally

Previously, drag placeholder would be removed in the call to
`tiling_layer.drop_window()` when dropping onto a tiling layer, but
would not be removed when dropping to a floating layer. Which would
leave a placeholder taking up space, and cause a panic on a future drag
operation.

Instead, call `cleanup_drag()` regardless, after `drop_window()`, to do
any cleanup that is still needed. This moves the call that was
previously added in 67d0a825.

src/shell/grabs/moving.rs

@@ -906,6 +906,12 @@ impl Drop for MoveGrab {
                         }
                     }
                 } else {
+                    None
+                }
+            } else {
+                None
+            };
+
             let mut shell = state.common.shell.write();
             shell
                 .workspaces
@@ -914,11 +920,7 @@ impl Drop for MoveGrab {
                 .tiling_layer
                 .cleanup_drag();
             shell.set_overview_mode(None, state.common.event_loop_handle.clone());
-                    None
+            drop(shell);
-                }
-            } else {
-                None
-            };
 
             {
                 let cursor_state = seat.user_data().get::<CursorState>().unwrap();