[Fleet] Clean up is_default and is_default_fleet_server flags

[juliaElastic]

While implementing #108456 we removed the creation of default policies as part of startup.
The intention was to remove the concept of default policies as well.

However, it turned out there is a functionality in Elastic Agent that allows an agent (fleet server) to enroll to Fleet without specifying a policy_id. In this case, Agent is looking for a default policy.
There is an issue raised in Agent to change the logic: elastic/beats#29774
When that change is made, it will be safe to remove the default flags from Fleet code.

Dependencies / blockers:

• elastic-package uses default flags to preconfigure policies: Predefine agent policies in the Kibana config elastic-package#655 (comment)
• [Agent] Change logic to find agent policy during Fleet enrollment beats#29774
• ECK uses default flags as well: Adjust Fleet recipes for default policy change cloud-on-k8s#5281

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)