E2E test: don't wait for page to load to edit MR

What does this MR do and why?

Similar fix to !120070 (merged) but in a subsequent action.

We don't need to wait for the merge check to be completed before clicking edit. In this MR we don't wait for pipeline spinner to complete before clicking Edit button whilst on an MR.

Resolves: #403189 (comment 1411188690)

The spinner occasionally doesn't finish due to known issue: #398559 (closed)

How to set up and validate locally

bundle exec rspec qa/specs/features/ee/browser_ui/3_create/repository/assign_code_owners_spec.rb

MR acceptance checklist

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