Derive secrets from matrix_homeserver_generic_secret_key, not matrix_synapse_macaroon_secret_key

We're trying to move away from implementation-specific variables,
hoping for a clean (implementation-neutral) examples/vars.yml file.

roles/matrix-dendrite/defaults/main.yml

@@ -63,8 +63,6 @@ matrix_dendrite_systemd_wanted_services_list: []
 # matrix_dendrite_template_dendrite_config: "{{ playbook_dir }}/inventory/host_vars/<host>/dendrite.yaml.j2"
 matrix_dendrite_template_dendrite_config: "{{ role_path }}/templates/dendrite/dendrite.yaml.j2"
 
-# A secret used to derive various other secrets
-matrix_dendrite_generic_secret_key: ''
 matrix_dendrite_registration_shared_secret: ''
 matrix_dendrite_allow_guest_access: false
 

roles/matrix-dendrite/tasks/validate_config.yml

@@ -5,7 +5,6 @@
       You need to define a required configuration setting (`{{ item }}`) for using Dendrite.
   when: "vars[item] == ''"
   with_items:
-    - "matrix_dendrite_generic_secret_key"
     - "matrix_dendrite_registration_shared_secret"
 
 - name: (Deprecation) Catch and report renamed settings