Wait for requests in `add_note` feature spec helper

What does this MR do?

This MR might make some feature specs less flaky, for example ./spec/features/projects/commit/user_comments_on_commit_spec.rb (gitlab-org/quality/triage-reports#1860 (closed)).

Also, it removes a bunch unnecessary wait_for_requests after calling add_note.

Spotted during !54215 (comment 509313009).

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content