Some archaeology to find out why there is configs about designer_coords that did disappear

[williamdes]

In phpMyAdmin versions before 4.3.0 alpha 1 there was a setting named
$cfg['Servers'][$i]['designer_coords']
See: https://github.com/phpmyadmin/phpmyadmin/blob/RELEASE_4_2_13_3/libraries/config.default.php#L399
See: https://github.com/phpmyadmin/phpmyadmin/blob/RELEASE_4_2_13_3/doc/config.rst

It no longer exists since #1232

The feature was first documented in fb9a3a5

[williamdes]

I am adding this back to the documentation so our users can remove it from their configs and find out it does not exist anymore.

cc @MauricioFauth for #17050
cc @clementmoulin to remove it from your config (found while working on #16906)

[williamdes]

I think the migration SQL script to 4.3 should have been adjusted, but sincerely, 7 years after, It will stay as it is.