[Workspace] Delete the virtual global workspace

[yubonluo]

Is your feature request related to a problem? Please describe.
Delete the virtual global workspace.
When workspace feature flag is enabled and user has not created any workspace, only OSD admin user can see any legacy saved object (not belonging to any workspace) on the home page. Non OSD admin users should not see the legacy saved objects if they can not access any workspace.

Describe the solution you'd like

A clear and concise description of what you want to happen.

Describe alternatives you've considered

A clear and concise description of any alternative solutions or features you've considered.

Additional context
#4944