Remove mention of unstructured logs like application.log in the documentation.

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Unstructured logs got disabled by default in 15.10 and removed in 15.11. However, we still have references to these logs in our documentation. For example:

• https://docs.gitlab.com/ee/administration/auth/ldap/ldap-troubleshooting.html (mentioned application.log under GitLab logs
• https://docs.gitlab.com/ee/administration/logs/#applicationlog

Ticket: https://gitlab.zendesk.com/agent/tickets/398711