Remove old praefect config virtual_storage_name and nodes

Once omnibus-gitlab!3754 (merged) and gdk is updated with the change to allow multiple praefect virtual storages, we can remove the backwards compatible support in praefect.

also we need to remove the text fixture in internal/praefect/config/testdata/single-virtual-storage.config.toml

Edited Nov 22, 2019 by John Cai

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