Remove instance integration models

What does this MR do and why?

This MR removes instance integration models located under Integrations::Instance namespace, since we no longer plan to use them. These models (and the corresponding instance_integrations table) were never used. We initially wanted to split integrations table into instance/non-instance integrations in order to shard them, but decided to go with a different approach instead (to add org_id to integrations table instead). Hence, these models are no longer required and should not confuse anyone on why these models are present.

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