Fix ghost Docker nodes causing 60-second VNC timeout on variable updates

When a Docker node is deleted, the compute node's DELETE endpoint only calls
node.delete() which removes the working directory but does not remove the node
object from the project's self._nodes collection. This causes ghost nodes to
remain in memory.

When project variables are updated, the code iterates through ALL nodes in
memory and calls update() on them. For ghost nodes with VNC configuration, this
triggers VNC startup attempts, resulting in 60-second timeouts waiting for X11
socket files that don't exist.

The fix adds await node.project.remove_node(node) to ensure the node object is
removed from the project's node collection when deleted, matching the behavior
of other node types that use manager.delete_node() which already calls
project.remove_node().

This resolves the issue where updating project variables after deleting a VNC
Docker container would timeout with: 'x11 socket file "/tmp/.X11-unix/X100"
does not exist'

Fixes issue #2755

gns3server/api/routes/compute/docker_nodes.py

@@ -235,6 +235,7 @@ async def delete_docker_node(node: DockerVM = Depends(dep_node)) -> None:
     """
 
     await node.delete()
+    await node.project.remove_node(node)
 
 
 @router.post(