Remove feature flag that holds 'JIT user set to observer' at creation / first SSO sync

[zayhanlon]

Follow up to #8411

Story

As the developer of the fleet server/agent
I would like to minimize features set behind features flag and automate the logic for role settings
So that I can maintain the code base with fewer ifs and buts.

As a Fleet admin,
I want to allow SSO to automatically update my users' role
So that I can manage my users in one place (my IDP).

Requirements

• If the SSO doesn't tell us a role at creation, then assume Global observer
• If the user already exists and no role coming in from SSO, no change to the existing permissions
• Remove feature flag that holds JIT user set to observer at creation

[lukeheath]

@zayhanlon To make this a proper story, I've added the story label. Would you please add the user story format? As a ___, I want ___ so I can ____. All user stories should have this at a minimum. Thanks!