There are no *.key files on recent Android devices, such as those running Oreo or Pie, but there is still the lockscreen.db database under /data/system.

Here is how to remove the PIN or passwords for making changes in this database:

adb shell
su
cd /data/system
sqlite3 locksettings.db
update locksettings set value=0 where name='lockscreen.password_salt';
update locksettings set value=0 where name='sp-handle';
.quit

Reboot the device and the screen lock will be removed.