Remove unused VERTEX_AI_PROJECT reference
What does this MR do and why?
- Remove reference to VERTEX_AI_PROJECT as it is not used anymore, and is not required for running chat_real_requests_spec.
- Remove ai_embedding_fixture RSpec metadata
MR acceptance checklist
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How to set up and validate locally
Run AI_GATEWAY_URL=http://0.0.0.0:5052 REAL_AI_REQUEST=true bundle exec rspec './ee/spec/lib/gitlab/llm/completions/chat_real_requests_spec.rb'
and it should work (provided AI Gateway is working correctly).