Remove deprecated remote_development_agent_config references

Issue: Remove deprecated API references to remote_deve... (#480769 - closed)

What does this MR do and why?

This is a followup to Switch from remote_development_agent_configs ta... (#480135 - closed) where we renamed the table to workspaces_agent_config.

With this MR we are removing all references for remote_development_agent_config since it is not used anymore.

References

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• Make workspace agent config types alpha (!166448 - merged)
• Delete remote_development_agent_configs table (!164032 - merged)

MR acceptance checklist

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• Remaining code related to remote_development_agent_config API references is deleted.
• Search for any remaining doc references across various doc repos
• Rename any old references if required - e.g. !163846 (comment 2079457470)

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Validate agent updates still work

• In local dev environment, ensure you have an agent set up and enabled for remote development: https://gitlab.com/gitlab-org/workspaces/gitlab-workspaces-docs/-/blob/main/doc/local-development-environment-setup.md
• Open the rails console bin/rails c and search for the agent config RemoteDevelopment::WorkspacesAgentConfig.last
• Update the workspaces_quota in the config.yaml file in your agent configuration project or through the rails console
• Navigate to http://gdk.test:3000/-/remote_development/workspaces/ and create a new workspace
• In the rails console check that the new workspace has the updated agent configurations

  reload!
  
  RemoteDevelopment::Workspace.last.workspaces_agent_config

Validate workspace creation and reconcile still work

• In local dev environment, ensure you have an agent set up and enabled for remote development: https://gitlab.com/gitlab-org/workspaces/gitlab-workspaces-docs/-/blob/main/doc/local-development-environment-setup.md
• Do a full-lifecycle test of creating, using, and terminating a workspace.
• You can use scripts/remote_development/run-e2e-tests.sh for this too, just make sure you don't have any spinning hung workspaces, or that QA test won't work.