Do not create a host file for the entrypoint's PHP session config

The role created ~/podman-data/nextcloud/redis-session.ini and the unit bind-mounted
it over the image path, purely because the entrypoint cannot write there itself.
Removed: the task, the mount, and -- because it is the only thing that triggers the
write -- REDIS_HOST from the environment file. The entrypoint now skips
configure_redis_session entirely and starts.

The Redis attach stays. The valkey role still puts nextcloud_net on the cache
container with the alias `redis`, so an instance whose config.php names that host
reaches it. Writing that configuration is config.php's business, and the readme
gives the three `occ config:system:set` lines for anyone who wants it, including
that this playbook will not do it for them.

The only thing lost is PHP session storage, which falls back to the container's own
filesystem -- a redeploy logs users out. That is the trade for not shipping a file
whose sole purpose is to work around a directory the image owns as root.

The env template and the valkey role comment both say why REDIS_HOST is absent, so
that adding it back is recognisable as the regression it is.

nextcloud/roles/valkey/tasks/main.yaml

@@ -8,10 +8,9 @@
 # asserts that nextcloud.network already exists, and notifies the "Restart
 # Valkey" handler defined in this role.
 #
-# `redis` is the alias the app connects to, not the container name: REDIS_HOST is
-# what the image writes the PHP session handler from, and what this play puts in
-# config.php's Redis configuration on an unattended install. Renaming it would
-# mean editing a live instance's configuration for nothing.
+# `redis` is the alias the app connects to, not the container name: it is the host
+# an instance configured for Redis has in config.php. This playbook does not write
+# that configuration, it only makes the name resolvable.
 
 - name: Attach Nextcloud to Valkey
   ansible.builtin.include_tasks: "{{ playbook_dir }}/../valkey/roles/attach/tasks/main.yaml"