To all the answers: hit Control ⌃ + Shift ⇧ + Delete ⌫
It is an editor shortcut for “quick delete entire line”
It does not add/modify/delete names from the list of login names that are suggested for autofill.
Hitting CONTROL-SHIFT-DELETE while highlighting the item you want to remove will remove that item from the dropdown window as seen here:
The Ctrl+Shift+Delete keys worked for me!