Remove capybara specs relying on jQuery.ajax

What does this MR do?

We had some capybara specs that checked whether expanding a diff calls the Backend more than once. These specs have been broken since we are using axios. Unfortunately as they are testing a negative, they never failed.

We are removing those specs in favor of new JS unit tests which actually test that the API is called exactly once.

Does this MR meet the acceptance criteria?

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