Default matrix_media_repo_generate_signing_key to false in the matrix-media-repo role

No need to duplicate the same logic as in `group_vars/matrix_servers`.

Having it disabled by default in the role itself and overriding it at the playbook level (based on the selected homeserver implementation) makes more sense.

roles/custom/matrix-media-repo/defaults/main.yml

@@ -320,7 +320,8 @@ matrix_media_repo_homeservers_auto: []
 matrix_media_repo_homeservers_additional: []
 
 # If true, the playbook will generate a signing key when the setup role is invoked.
-matrix_media_repo_generate_signing_key: "{{ matrix_homeserver_implementation == 'synapse' or matrix_homeserver_implementation == 'dendrite' }}"
+# This is adjusted in `group_vars/matrix_servers` depending on the homeserver implementation.
+matrix_media_repo_generate_signing_key: false
 
 # Path where the homeserver signing key is located. Set automatically in
 # `group_vars/matrix_servers` depending on which homeserver is being used.