Clean up schema for table historical_data

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From https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):

• Wrong data type: created_at should be timestamptz
• Wrong data type: updated_at should be timestamptz
• Missing constraint: created_at NOT NULL
• Missing constraint: updated_at NOT NULL
• Missing constraint: active_user_count NOT NULL
• Missing constraint: date UNIQUE (1 entry violates this rule- 2016-07-15)

What is this table even used for, and do we need it in the first place?