Revert "Merge branch '118597-improve-autofill-suggestions-for-usernames' into 'master'"

Reverts !54072 (merged), which seems like the most likely cause of #321925 (closed).

The bug is causing a QA test to fail and blocking deployments gitlab-com/gl-infra/production#3647 (closed)

Confirmed on GDK that this revert resolves the problem and allows the QA test to pass.