"Remember username" checkbox always being rendered regardless of the $CFG->rememberusername setting

To reproduce:

1. Log in as an admin
2. Go to "Site administration > Security > Site security settings"
3. Set "Remember username" to No
4. Log out (or open an incognito browser window)
5. Go to the login page.
6. Expected: The "Remember username" checkbox shouldn't be rendered.
7. Actual: The "Remember username" checkbox is still being rendered.