Chore: drop vertex_gitlab_docs db

What does this MR do and why?

The embeddings db no longer is in use, so we can delete the vertex embeddings db to make sure we solely use Google Vertex db

Changelog: removed EE: true

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Access the gitlabhq_developemnt_embedding database with gdk psql
2. Drop the table with bundle exec rake db:migrate:up:embedding VERSION=20240608215318
3. Make sure the following vertex_gitlab_docs table no longer exists.

Explanation of DIFF

This migration drops the vertex_gitlab_docs table. The table is no longer used and can be safely removed.