Spec: remove unnecessary visit in edit MR spec

What does this MR do and why?

Resolves #418105 (closed)

visit edit_project_merge_request_path(target_project, merge_request) (and surrounding block) isn't necessary for these tests because the tests are already on the edit page at this point.

This is because spec/support/shared_contexts/merge_request_edit_shared_context.rb visits the same page, it was effectively refreshing the page, and thus causing an alert.

How to set up and validate locally

rspec spec/features/merge_request/user_edits_mr_spec.rb

MR acceptance checklist

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