Remove alert entity and related code

What does this MR do and why?

Remove alert presenters and alert entity

Update/remove spec files

Screenshots or screen recordings

The removed code was used in Dashboard Operations entity but it was not displayed on the FE anywhere. No visual changes.

Before	After

How to set up and validate locally

1. Operations dashboard response should not have last_alert anymore.
2. Everything else should just work and pipeline should be 💚

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #397138 (closed)