Let Docker allocate the network subnet again

Reverts the explicit subnets: the pools were freed by deleting five networks, so
the network role is back to its single idempotent task - ensure the network
exists and Caddy is on it - and the Sites no longer declare a subnet. The
deployment readme keeps the note that a new Site needs a config, and therefore a
new tag, but no longer asks for a free /24.

If the pools fill up again the failure is recognisable: 'all predefined address
pools have been fully subnetted', and the choices are deleting stale networks or
declaring a subnet per Site.

ansible/vars/hngmohali.yml

@@ -11,6 +11,3 @@ docker_network: '{{ docker_container }}_net'
 # Everything the app writes — data/ and the untracked site.env. Deliberately the
 # directory the old static menus were served from.
 app_root: '/var/lib/{{ host }}'
-
-# The host's default Docker pools are exhausted, so every Site names a subnet.
-network_subnet: 10.210.3.0/24