Drop the instance_double of a deleted CloudConnector class

What does this MR do and why?

ee/spec/lib/gitlab/llm/ai_gateway/client_spec.rb declared a lazy let(:service) built on CloudConnector::BaseAvailableServiceData, a class removed in !192366 (merged). Nothing referenced the let, so it never resolved, but the first example to use it would have failed with a NameError. This MR removes it, along with a RSpec/MultipleMemoizedHelpers disable that the smaller helper count no longer needs (rubocop reports it as redundant once the let is gone).

References

• Resolves #628838 (closed)

Differences

Before: A let pointing at a deleted class.

After: Removed; no behaviour change.

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