Use deleted tables/views dictionary to fetch gitlab schemas

What does this MR do and why?

In this merge request, we are deprecating the Gitlab::Database::GitlabSchema::DeletedTables constant in favor of the database dictionary.

When a table is deleted, the developer needs to move the table dictionary file to /docs/deleted_tables/ directory.

Closes gitlab-org/database-team/team-tasks#301 (closed)

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