Delete object_state_models.md

doc/development/object_state_models.md

-# Object state models
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-## Diagrams
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-[GitLab object state models](https://drive.google.com/drive/u/3/folders/0B5tDlHAM4iZINmpvYlJXcDVqMGc)
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-## Legend
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-![legend](img/state-model-legend.png)
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-
-## Issue
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-[`app/models/issue.rb`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/models/issue.rb)
-![issue](img/state-model-issue.png)
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-## Merge request
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-[`app/models/merge_request.rb`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/models/merge_request.rb)
-![merge request](img/state-model-merge-request.png)
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