Change feature spec to use data-qa and remove unused js- qa selector

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The following discussion from !46406 (merged) should be addressed:

• I'd love to see this changed to a data-qa selector, as I think that's what is expected by the Quality team

• Also, if we're not using js-file-title here maybe it could be removed.

spec/features/merge_request/user_views_diffs_spec.rb

  it 'expands all diffs' do
    first('.diff-toggle-caret').click