Remove workspaces_agents_availability_admin feature flag

MRs:

Draft: Workspaces Cluster Agents Admin Setting UI (!194264 - closed)

Cleanup feature flag, add feature spec for inst... (!195665 - merged)

Description

The workspaces_agents_availability_admin WIP feature flag was created to iteratively implement the organizations Workspaces Cluster Agents Admin Setting UI. Now that the feature is complete, we can remove this flag.

Acceptance criteria

• Perform final UI test on the feature overall.
• Once the above is good, remove WIP workspaces_agents_availability_admin feature flag so that users can start using this feature 🎉
• Remove the flag from chatops by running this command: /chatops run feature delete workspaces_agents_availability_admin