[Fleet][Agentless] Fleet should default to agentless deployment mode for custom integration

[nchaulet]

Description

When developing an integration, we upload the integration this create a custom integration that is not deployable to agentless, unless the config xpack.fleet.agentless.customIntegrations.enabled: true is provided.

We already hide the deployment mode selector, but it seems we still select agentless as a default mode

It seems the relevant code to change is somewhere here

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)