whatsapp bridge: disable logging to external files

The same logs still go to the systemd journal.

roles/matrix-bridge-mautrix-whatsapp/templates/config.yaml.j2

@@ -211,7 +211,8 @@ logging:
     # The directory for log files. Will be created if not found.
     directory: ./logs
     # Available variables: .Date for the file date and .Index for different log files on the same day.
-    file_name_format: "{{ '{{.Date}}-{{.Index}}.log' }}"
+    # empy/null = journal logging only
+    file_name_format:
     # Date format for file names in the Go time format: https://golang.org/pkg/time/#pkg-constants
     file_date_format: "2006-01-02"
     # Log file permissions.