E2E test: only close popup once when merging

What does this MR do and why?

Since merge! immediately calls try_to_merge! we only need to close the rich text popup once in try_to_merge!

Resolves #418873 (closed) as it was trying to close the popup twice.

How to set up and validate locally

bundle exec rspec qa/specs/features/browser_ui/3_create/merge_request/rebase_merge_request_spec.rb

MR acceptance checklist

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