Remove nav2 demo (issue #258)

The nav2 demo has been relocated to the [demos repo](https://github.com/space-ros/demos/tree/main/nav2_demo).

Author: Bckempa

README.md

@@ -6,7 +6,6 @@ The projects in this repository are applications and demonstrations built on the
 See individual template directories for details.
 
 * [moveit2](./moveit2)
-* [nav2_demo](./nav2_demo)
 * [navigation2](./navigation2)
 * [renode_rcc](./renode_rcc)
 * [rtems](./rtems)